Having read Epic's filing and Apple's response... Epic has bad lawyers.
Based on the the complaint and the response, Epic has a weak case.
First off, there's the issue of filing for the court to force Apple to reinstate Fortnite onto the app store. Apple countered by showing that they offered Epic the chance to submit a version without the IAP workaround, would put it back on the store. They could still file the anti-trust suit, and could reclaim any money lost during the time of the lawsuit as part of their damages if successful.
Apple's case seems as far as I can see 100% valid. Epic could have chosen this route, and still could, and there doesn't seem to be any benefit to doing it the way they did. Not only is their restraining order part of the suit kind of dumb, but it's also not a good look and will likely prejudice a judge/jury against them.
As for the substance of the claim, it's basically that they are accusing Apple of leveraging the App Store's power to force the use of Apple Pay. Legally, that's not a good claim for reasons that Apple spells out in their response. I'd have to look into whether the sources they cite are being interpreted correctly (naturally Apple is biased), but they make a good case. Epic on the other hand... cited no actual case law or court decisions, and their complaint actually made me cringe a couple of times, as they tried to portray Fortnite more as a public service than a game.
All of this is not to say that Apple's policies are necessarily good, but Epic's lawyers did a shit job making the argument that they're monopolistic.