Neither Apple nor Google is a monopoly in mobile games. By revenue Apple has about 60%. Check [1], search threshold to see what cases often require.
Taken together they would be, but then you cannot prosecute Apple and win without showing collusion between them and prosecute them both for collusion.
[1] https://www.justice.gov/atr/competition-and-monopoly-single-...
Apple has signaled that Epic is welcome to submit a version of their app which does not violate their rules, and they will approve that. They're walking a fine line, but I think their legal argument is sound on that score.
We can agree or disagree about the overall right or wrong of Apple's rules, that's separate from the issue of Epic's motion, which is absolute nonsense.