That's probably the pitch they're given. I doubt it's actually what happens.
For example, The Russo Brothers worked for Marvel for quite a while. They directed Cherry, and in their own words:
They are now directing... a $200m action-thriller franchise starring Ryan Reynolds. I don't imagine this is a passion project.
Since Jon Favreau directed Iron Man, he's directed four other big-budget studio movies. He's done one 'passion project', Chef, which cost 11m. That's not too different than the 5m he could get for an independent movie in 2001. Colin Trevorrow has not done any passion projects since he got involved with Jurassic World and Star Wars. Rian Johnson had directed 30m Looper and then directed 40m Knives Out. I don't think Last Jedi had much to do with that. Joss Whedon, who was already a well-known television creator, got involved with Avengers and DC for a while, and got... another tv show greenlit. Chloe Zhao, who won a Best Director oscar, finished Eternals and is now doing... a sci-fi western about Dracula. I doubt this was her calling. Even someone like Kenneth Branagh, who was already a respected director, has spent the bulk of the last decade directing big budget studio stuff. In exchange for what, Belfast, which barely cost any money?
This career path works best when you want to make big-budget movies in the first place. If you're Christopher Nolan, you want to make big movies, whether they have Batman in them or not. So it works for you. Ditto if you're Zach Snyder. But if you want to make something else and are only getting into Marvel/DC stuff for reputation, it's probably not going to pay off.
Also it should be said that a lot of these people are talented, but you need to hone your skills. If you spend your formative years directing $100m big budget movies where a lot of the work is done for you, you're not going to have the skills to make these passion projects.