According to which legal practitioner? Youtube did describe themselves as a "public forum" but that doesn't mean they legally are one. They're still a private company that can control whatever content they want on the platform.
I don't like Youtube's biased censorship and content moderation as much as the next person, but at the end of the day they're a private company. There's not much you can do. Even with a strained reading of the Pruneyard and Packingham cases, it's not possible to legally define Youtube, Google, or other social media companies as state actors, and therefore it's impossible to define their platforms as public forums.