I would suspect being unable to update the iOS versions of Fortnite might, oddly enough, be linked to it having been removed from the app store and their account being banned. If their account were reinstated and Fortnite allowed back into the store, I don't know what technical hurdles would remain preventing them from updating it.

edit: Sweeney wrote: "Though we can't update the Fortnite version that users still have on their iOS devices, we've disabled Epic payments server-side[...]". This reads to me that he's stating how things are currently. That, they don't have a mechanism to update Fornite. Doesn't even read that they won't, if able to.