I suppose you are right in the sense that YouTube is certainly mainstream. But when people refer to mainstream media in the U.S., they are usually referring to the popular TV News outlets that have dominated for many years. CNN, FOX NEWS, MSNBC, Etc. I think they are referring to "independent" as any news outlet that is not corporate owned. These independent outlets are podcasts, not television productions, that can (but don't need to) use YouTube. Joe Rogan's deal with Spotify proved that (part of the deal is he can't post his podcasts on Youtube in their entirety, only small clips). As a podcast, they do not have anyone censoring what they say. They do not have to pander to advertisers or politicians. They can survive with a small paid audience that supports them directly. Also, BTW, just because someone is independent, doesn't mean they are not completely full of S*&%. That being said, YouTube is mainstream for sure and they certainly do censor people.
Youtube's revenue model does indeed significantly disadvantage small, independent channels as well as content Youtube's management deems "controversial" such as any nonfictional content related to homosexuality or same-sex relationships, transgender content, strong language, imagery of graphic violence of the kind one would see in a historical documentary, and of course radical leftist politics.
Of course, calling them "independent" channels is a complete misnomer in the first place - many of these creators depend on Youtube for their audience and - both directly and indirectly - much of their revenue. Note that this dependency tends to hit those creators the hardest who actually go the extra mile and produce high quality audiovisual content such as nonfiction documentaries, and so have a rather limited selection in platforms to choose from. Of course, pod cast series - especially those like Rogan's which require zero research and minimal effort - have a much wider potential way to seek audiences independently of a single platform. Rogan himself, of course, is no less "mainstream" than many other radio show hosts with a similarly-sized audience, neither in his content nor the audience he tends to attract.
The overwhelming number of these shows are, by the way, sponsored to some degree or another - claiming that they would not have to "pander to advertisers" is just factually wrong, and also belies a fundamental misunderstanding of the economics of producing media content. "Content production", whether it is mindless clickbait or high quality journalistic work, is an economic activity like any other, and such has to pay for itself - or ideally, generate a profit; at the very least, creators have to be able to financially maintain themselves to the degree that they can for their studio and/or recording equipment, pre and post production, professional staff, and in many cases, also guest appearances, and in the case of journalistic content, even the occasional legal protection.
In other words, somebody gotta pay for all this. That's where all the sponsorship deals, ad revenue, and wide ranging corporate influence tends to come in. And most content creators on online platforms tend to be just as dependent on these income channels as - or in some cases, arguably moreso than - the vaunted "MSM".
*) Sidestepping, for a moment, that all Western free speech legislation operates under the liberal-capitalist conception of censorship, where the government interferes in otherwise "natural" capitalist control over media content
