That might be overstating it, as this is coming secondhand from a WSJ story and not from the Russos themselves, but still. “People familiar with the matter” believe that there are at least some concerns from them. However, unlike with Scarlett Johansson, whose Marvel career was pretty much done after Black Widow anyway, there’s no reason to think the Russos wouldn’t want to come back and make another Avengers or whatever someday. After all, those last two movies were kind of a big deal, both culturally and financially, so losing the Russos could have a bigger impact on Marvel’s future than losing Scarlett Johansson’s goodwill.
Again, though, this could all be making a mountain out of an Ant-Man hill. Maybe it’s not a big deal, maybe these discussions weren’t especially serious, we don’t know. What we do know is that speculation about comic book movies is always fun, so let’s talk about what hypothetical Marvel movie the Russos are supposedly talking about. They’re noted fans of the Secret Wars storyline from the ‘80s, in which a disco space god called the Beyonder gathers up a bunch of heroes and villains and has them fight to the death on his Battleworld, with Joe Russo joking a couple of years ago that they’d probably need another six-film series to cover that whole saga. (Not to mention the fact that Secret Wars prominently featured a couple of super-teams called The X-Men and The Fantastic Four who haven’t shown up in the MCU yet.)
So that seems unlikely, but then again, Secret Wars writer (and former Marvel EIC) Jim Shooter recently claimed that Marvel tried to get him to sign a contract that—through some legal maneuvering—would’ve resulted him in handing over any rights he had to Secret Wars. His theory was that Marvel was trying to cover its bases for a potential Secret Wars movie, which makes a bit of sense, but then again again, Marvel Studios has never let any Big Plans like that leak this easily in the past. So we still don’t know much of anything. It’s fun to be back in this world where Marvel’s future isn’t totally obvious though, right?