You cannot use your own IAP system. You can link to a webpage, but Apple is still entitled to the 30% cut and can bill the developer after the fact each month or so for the commission that would have been paid.
And for this reason, Epic owes 30% of their unpaid iOS IAP revenue to Apple (about $4 million) for breach of contract and Apple does not need to let Fortnite back on the store.
Epic won almost nothing. 9/10 counts lost, and the remaining count they won still owes 30% but it would be collected through a different method. Super shallow victory that.