The confusion Jeremy Crawford's initial Dragon's Breath ruling caused is a great example of why the decision was made to retcon all rulings from before 2019, leaving the Sage Advice Compendium as the sole source of official rulings going forward.

The confusion caused by Jeremy Crawford's initial Dragon's Breath ruling is a great example of why the decision was made to retcon all rulings from before 2019, leaving the Sage Advice Compendium as the sole source of official rulings going forward.

[This answer has been edited to reflect recent guidance on Twinned Spell. This answer originally was "You can twin the spell as long as you don't target an object."]

RAW and RAI, the spell needs to target exactly one creature and cannot be capable of target anything else - including objects - in order for Twinned Spell to work.

This answer has been edited to reflect recent guidance on Twinned Spell. This answer originally was: "You can twin the spell as long as you don't target an object."

Your first interpretation is correct, with some technicality.

RAW and RAI, the spell needs to target exactly one creature and cannot be capable of targeting anything else - including objects - in order for Twinned Spell to work.

The spell needs to target exactly one creature in order for twin to work...

... but there are caveats.

Nystul's Magic Aura, on the other hand, can only target one creature, but it can also target an object, and that's where the complication comes from.

According to this question about the Heat Metal spell, spells that target objects are ineligible to be twinned. Particularly because objects are not creatures, and the Twinned Spell metamagic option explicitly makes mention of "creature" and not "object."

This leaves us in an interesting predicament, as Nystul's Magic Aura fulfills the requirement of only being capable of targeting one creature, but also that it can instead target an object.

This DM would interpret that to mean that if you target a creature with Nystul's Magic Aura, you could then twin the spell and choose a new creature to also be affected. However, if you choose an object as your original target, you could not then twin the spell.

A few words on the podcast (and subsequent rules interpretation) you mention:

(NB: The question's original phrasing placed significantly more emphasis on the podcast and the "ruling" it presented. This section of the answer specifically addresses that portion.)

We can find the 2019 SAC here. I searched for mentions of "target," "Nystul," and "affect." I read through all the occurrences of the aforementioned words. Unless I missed something, the SAC does not currently appear to define anything in regard to spells automatically targeting things they affect. Where there might be exceptions, they seem to be explicitly covered in the description for a given spell. Funny enough, Fireball is one such spell. It initially targets a point in space and then treats the affected creatures like targets (emphasis mine):

"The target" in this case is explicitly defined. Later, if another spell targets the same target, Nystul's defines how that other spell behaves. That is the extent of the interaction. We choose Nystul's targets when we cast the spell. We do not suddenly have additional targets when someone else targets our target. Nystul's does not make mention of the other spells becoming targets, so we cannot consider them to be.

The confusion Jeremy Crawford's initial Dragon's Breath ruling caused is a great example of why the decision was made to retcon all rulings from before 2019, leaving the Sage Advice Compendium as the sole source of official rulings.

[This answer has been edited to reflect recent guidance on Twinned Spell. This answer originally was "You can twin the spell as long as you don't target an object."]

RAW and RAI, the spell needs to target exactly one creature and cannot be capable of target anything else - including objects - in order for Twinned Spell to work.

Nystul's Magic Aura, on the other hand, while it does only target one creature, can also target an object.

Apparently, according to recommended guidance in a 2020 version of the Sage Advice Compendium, spells that can target objects are ineligible to be twinned:

If you ... are still unsure whether a particular spell qualifies for Twinned Spell, consult with your DM, who has the final say. If the two of you are curious about our design intent, here is the list of things that disqualify a spell for us:

• The spell has a range of self.
• The spell can target an object.
  [...]

-- Sage Advice Compendium, page 6

According to this guidance, Nystul's Magic Aura cannot be twinned because it can target an object, and that's that.

But I personally think that's a bit dumb.

This DM has allowed - and will continue to allow - spells like Fire Bolt to be twinned. I interpret the rules to mean that if you target a creature with a spell, you could then twin the spell and choose a new creature to also be affected; but if you choose an object as your original target, you could not then twin that spell.

An aside about targeting and that podcast:

I searched the SAC, and unless I missed something, the SAC does not currently appear to define anything in regard to spells automatically targeting things they affect. Where there might be exceptions, they seem to be explicitly covered in the description for a given spell. Fireball is actually one such spell. It initially targets a point in space and then treats the affected creatures like targets (emphasis mine):

"The target" in this case is explicitly defined. Later, if another spell targets the same target, Nystul's defines how that other spell behaves. That is the extent of the interaction. We choose Nystul's targets when we cast the spell. We do not suddenly have additional targets when someone else targets our target. Nystul's does not mention the other spells becoming targets, so we cannot consider them to so be.

The confusion Jeremy Crawford's initial Dragon's Breath ruling caused is a great example of why the decision was made to retcon all rulings from before 2019, leaving the Sage Advice Compendium as the sole source of official rulings going forward.