Yes, you've missed a lot apparently.
The first correct thing you've acknowledged is that you know they are both seperate, therefore you admit you were incorrect when you claimed "honda and acura are not" different brands. They cannot be both seperate and not be different brands.
Also, where are you getting this one different car from? And what is the basis, are you clamining Acura only make one different car to Honda or Honda to Acura? This is factually not the case either way. They've made plenty of different models to one another for long enough.
That Acura wouldn't exist without Honda is moot, that is not a factor in determining if two seperate companies are the same company or brand. But then you've also confirmed you now accept they are different brands, so this debate is proving to be a right whirlwind.
You are perfectly free to hold a personal opnion that it would be a waste of resources to include both brands becuase you feel most of the cars are too similar. That opinion is fine, however it can only ever be you're personal opinion on the matter as the series has, as you have acknowledged, included both brands plenty of times.
You cannot claim that they are the same brand when they factually are not or that they only have one unique vehicle between them when they don't or that because PD are a Japanese developer it makes no sense for them to do something that they've done since GT2.
Those are not opinions, they're just wrong on a factual level. But you can dislike the idea of including both brands, just don't dig a grave for yourself by making stuff up along the way next time.