That's why I feel like this idea is useless, even for the general population. Even with some sort of visual/audio based hashing, so that the hash is independant of minor changes like video resolution which don't change the content, and with major video sites implementing a way for the site to verify that hash matches one from a trustworthy keyserver equivalent...
The end result for anyone not downloading the videos and verifying it themselves is the equivalent of those old ”✅ safe ecommerce site, we swear" images. Any dedicated misinformation campaign will just fake it, and that will be enough for the people who would have believed the fake to begin with.