I took their take as meaning Superman didn’t have another superhero film in its immediate orbit prior to its debut. In other words, when Superman came out it was the first and only choice for people who love CBMs. By the time Fantastic Four arrived, audiences now had two choices. Now, let’s say a lot of those people took their families to see Superman more than once. They might have exhausted their budgets for July. And with F4 undoubtedly coming to Disney Plus within the next couple of months, they opted to sit F4 out.
Again, Fantastic Four did slightly better with RT critics and audiences than Superman and both got A- Cinemascores. So, something is off with the general audiences. We can’t simply write it off as “lack of interest.”
Personally, I think it’s the Disney Plus streaming option that’s biting them in the ass.