What separates Vaush and Destiny from terminally online Marxists is also that they support liberal causes without the "but the US can't be involved" shtick. "Why yes, I support gay rights in Saudi Arabia and revolution in Russia-it just needs to be a leninist revolution without Imperialist Americans causing it for imperialist reasons".
I've seen plenty of leftists that alienate themselves from the liberal mainstream with this attitude-mass culture is bad, the state department is bad, the CIA is near demonic Vaush responds with "lol nigga I support liberal causes regardless"-this gives him massive in roads with liberals that don't want to hear about American coups in Latin America or support of dictatorships or how the US is a reactionary force in the world-sure the US has a racist history and all, but we are fundamentally good guys, or getting better.
Vaush doesn't engage in constant "america bad" rhetoric or create fantasy scenarios where "good" social progress happens untainted by say state department backed NGOs or mass media support would lead to some alienated Trot or Maoist sect saying it's all crap. "Uh yeah, overthrow Putin, but it should be Russian workers (presumably under some as of yet uncreated communist party) not American led protesters"
This a perpetually self-defeating and alienating attitude-if you're a leftist in the US, your liberal friends are going to tune you out and you're left to discuss obscure Trot sects or Maoist larping in the void.
Same thing with domestic policy-leftists who go "I support gay rights and tranny-ism just no corporations that capitalist stuff is insincere and reinforces oppression".
I have seen leftists online with this attitude for years. Believe me it does not win them friends amongst normie libs.
Vaush and by extension Hasan and Destiny don't engage in this purity spiraling firing squad of self-induced ghettoization. They make liberals more comfortable with the label "socialist" and online socialists more comfortable cheering for NATO and stanning corporate blockbuster Marvel films.
They support liberalism and globohomo as it actually happens not with some sort of qualifier that it needs a Leninist vanguard party or must drape itself in the label of Marxist or anti Imperialist for it to be not American imperialism or something.
This along with their clear rhetorical training and "hip" presentation is why they have become (regrettably) so popular.
Regarding Mass Culture-bread tubers don't engage in the leftist critiques of "Marvel is brain dead corporate shlock that promotes the US military" and "video games are an opiate of the people that also propagandize the military"-Hollywood is bad, please watch obscure films and read even more obscure books. They like Marvel, they play vidya, they don't come at normie culture with an attitude of condescension or hostility-as a lot of alienated leftists do. This also wins them lots of favor and makes them appear "normal" and within the mainstream.