I agree I don't really think there's anything here besides compression algos being tested. At the very least, I'd need to see far far more evidence of filters being applied than what's been shared in the thread. But having worked at social media in the past I must correct you on one thing

>Automatically applying "flattery filters" to videos wouldn't significantly improve views, advertising revenue or cut costs.

You can't know this. Almost everything at YouTube is probably A/B tested heavily and many times you get very surprising results. Applying a filter could very well increase views and time spent on app enough to justify the cost.