What makes something 'magic' (official rulings)
The sage advice compendium gives a way of determining if something is magical under 'Is the breath weapon of a dragon magical?'
Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell
that’s mentioned in its description?
• Is it a spell attack?
• Does its description say it’s magical?
• Is it fueled by the use of spell slots?
If your answer to any of those questions is yes, the feature
That said lets point out a few things, If you have a magic item (lightbringer) that produces light, the light itself, is not a magic item, it doesn't specify that it is a spell and is not a spell attack, and the description of the light doesn't say it is magical, it is not fueled by the use of spell slots either. Given this the official ruling is that this effect is indeed not magical. Likewise, one may ask: The cantrip light causes an item to glow. Is the light (effect) caused by the spell a spell? Actually I did find evidence that suggests it is: What is a spell?, The section suggests that a spell is a discreet magical effect.
A genetic fallacy is when an argument or claim is dismissed or affirmed because of its origin or history. To say the effect is magical (claim) because it comes from a magic source (origin) is a genetic fallacy: affirming a claim because of its origin.
This fallacy might be avoided simply by the definition of magical. If the definition of magical were 'related to magic regardless of whether magic itself a component,' or 'created by magic regardless of whether magic itself is a component,' then there is no fallacy in the argument from the paragraph above. This would be technically strange though. It would implicate that magical light doesn't have magic as a component and thus has no magic in it, it was merely created from a different source than normal light. We don't have an example of natures laws behaving differently based on origin in the real world.
JC's interpretation of RAW
As an answer pointed out in the 'does lightbringer illuminate...' thread that I posted, Jeremy has made two tweets: tweet 1 tweet 2
Light from any magical source can illuminate the area of a darkness spell, but the darkness spell can dispel light created by a spell of 2nd level or lower, not light created by a non-spell.
If a source of magical light is not a spell of 2nd level or lower, darkness can be illuminated by that light.
This conflicts with the official rulings: The compendium is a list of official rulings. Note that it was pointed out to me that being an official ruling does not mean RAW.
Thought experiment (an implication)
From the official rulings, it appears that rarely an effect from something magical is a magical effect. A funny implication if this weren't the case is as follows: A level 1 feature from the artificer 'magical tinkering' can illuminate maddening darkness, a spell that can't be illuminated by another spell 8th level and under.
From the 'official rulings' of the SA compendium, effects from magic items aren't magical effects unless the description of the effect explicitly says that it is magical. A spell effect constitutes what the spell is, and therefore spell effects are magical. This is the official ruling, but as said, official rulings aren't necessarily RAW. Official rulings exist because of the need to interpret what RAW leaves vague. JC's interpretation runs contrary to the official rulings in the SA compendium, this might suggest that the official ruling in this scenario runs contrary to RAI.