Despite the rather clunky wording the logic of this is actually clear. Lets break it down:
While you're mounted, you have two options. You can either control the mount or allow it to act independently.
All this says is that there are 2 options, control or independent action. It does not actually say that you choose which one.
Intelligent creatures, such as dragons, act independently.
OK, so intelligent creatures like dragons (or your half-orc barbarian fighter acting as a mount for your halfling wizard c.f. Master Blaster) always act independently, you cannot control them. What counts as "intelligent" is a DM call; perhaps all "beasts" are unintelligent and everything else is intelligent or perhaps the ability to speak (or just understand) a language will be your criteria.
You can control a mount only if it has been trained to accept a rider.
So, if the mount is untrained it will always act independently. Only if it is both unintelligent and trained for riding do you get to choose.
Summarising in a logic table:
Situation | Intelligent | Unintelligent
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Trained | Acts independently | Rider chooses
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Untrained | Acts independently | Acts independently
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