It looks like it took social media shaming and backchannels, similar to the occasional desperate post on HN for support for locked accounts and such.

If you watch the video, they explain how 1) none of the videos that they pulled clips from to make the highlight reel were age restricted, 2) this exact situation occurred last year, and the youtube rep assured them this was a mistake and it wouldn't happen this time, 3) the Youtube rep once again reassured them that this was absurd, 4) after reaching out to the rep and being told their complaint was being reviewed, videos that were years old suddenly became age restricted, 5) the rep apologized and explained that the new age restrictions were as intended.

This means that even if you have 2 million subscribers and a youtube rep for support, asking google for help is just as likely to ruin you as actually fix anything.