NewPipe basically scrapes YouTube site, it doesn't use their API yet it's banned from Play Store. Kiwi Browser back then got warning it will be removed from Play Store if they don't remove option for background playback because of YouTube. There were multiple cases like this where Google either took down apps or warned developers because they enabled background playback on YouTube.The same workaround used by Brave, which is loading the desktop site to retrieve the video as a standard media object, works on Firefox, Vivaldi, etc. They're not abusing APIs.
You could guess that it has to do with integration with Brave Shields (privacy tools), the built-in option of rewarding YouTube creators, and First Amendment protections for adblocking—Garcia v. Google (2015) upheld users' rights to control what loads on their device. Brave obviously has quite a bit of weight compared to little apps.
Perhaps this apparent exception raises eyebrows. I won't lean too hard into speculation.
On the other side you have Brave that blocks ads on YouTube and advertises itself this way, has option for background playback and even has option to open all YouTube links directly in the browser. And Google is silent. You have to admit it's a little sketchy.

That can't be the reason. Brave literally replaced Google's ads with their own and didn't pay the creators if they didn't partner with them.The built-in option of rewarding YouTube creators...
