You don't need third-party APIs to make a YouTube viewer. There's a bunch of 3rd-party YouTube viewers like SmartTube, ReVanced, etc. that bypass ads and don't use the official API, plus of course the yt-dlp downloader.
I'd say the lesson here is NOT to rely on official APIs.
However, upon reading this blog post, it does seem he's using the official API so I guess he thinks he'll be fine as long as he doesn't block ads. Time will tell.