He's not knocking their performance. He's just saying that these roles aren't making them movie stars, the same way we had legitimate movie stars in the 80s, 90s, and even 2000's. They aren't even parlaying the massive success of their MCU fame into massive success outside of the MCU. Not to the extent that famous actors in yesteryear could. Some of these MCU actors are starring in straight to Netflix movies nobody has heard of when they aren't in a MCU flick.
Batman, Spiderman, and Superman have been played by a ton of actors, and will continue to be in the future, along with Ironman, Wolverine, Black Panther. People will have their favorite performances, like some folks prefer Michael Fassbenders Magneto to Ian Mckkellen, but the fact will remain that its the character that is the star....and this reality, which wasn't as strong when RDJ first blew up the role of Iron Man, is getting more pronounced by the year.
Movie stars used to be able to generate huge box office numbers on their names alone. People went to see the latest Arnold movie, or the latest Will Smith movie, or the latest Tom Cruise movie. It didn't really even matter what the movie was about, if it was original or based on something pre-existing. They just wanted to see their favorite movie stars in their next thing. And studios would greenlight movies based on the box office appeal of these stars. But that has shifted in the era of Marvel Studios and franchise film building.