I'm not sure what I think regarding this discussion if the actors/actresses of superhero movies are the stars or only the heroes they portray. In a lot of cases, it's been the actors portrayal of a certain character that made the whole thing work, be it Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Tom Hiddleston as Loki etc. Would all of these films really have been as succesful without the charisma of their lead actors? Could they have just recast Tony Stark at the start of Phase 3 and nobody would have cared since we all came to see Iron Man anyway, not specifically Robert Downey Jr. as Iron Man? I don't think so. In many cases, the actors have made stars out of the superheroes with their portrayal just as much as the films have made stars out of the actors.
And there is no denying imo that many superhero films have indeed made household names out of their lead actors in return, with Chris Hemsworth maybe being the most notable example.
Now, is Tarantino right when he says the fans don't follow their favorite actors to their other movies when they aren't portraying their superhero alter egos? Partially. I'd say they don't follow them blindly. I can only speak for myself, but when I read about some new movie and hear that it stars actors or actresses that I know and like from the MCU, for example, this will peak my interest and I will keep an eye out for that movie. So I would say it definitely helps a film if it has stars with a big fanbase in it. It's just not the only reason to watch a film for me. If everything else about the movie sounds or looks really crappy, I'll probably not watch it in theaters, at least. For example, yeah, if I hear Robert Downey Jr. is in a film, sure, I'll check it out, but if it turns out to be pretty bad like Dolittle was, pass.
But I think an actor most famous for playing a superhero character can definitely help attract an audience for his other films. Ryan Reynolds is a really good example for that. After Deadpool, he definitely gained a lot of new fans, and I absolutely think that was a benefit for films like Free Guy, which did pretty well for a comedy film that belonged to no particular franchise and was also released in the middle of the pandemic. The thing is though, that film had also other things going for it, i . e. it was a good comedy in general.
