I don't think you read the same email I did. He said they "can't", not that they "would not". His wording indicating that they are unable to, your wording indicating they choose not to.
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.
Elsewhere, Sweeney indicated they had no intention of publishing a new iOS version that complied with the current Apple rules, instead targeting a hypothetical future where alternative in-app purchasing was allowed by Apple.
Therefore, while they can't publish a new version at the moment because their account is disabled, they have zero intention of publishing a new version under the current iOS App Store rules.
The version they were talking about publishing is a new Mac App Store version. I'm curious if people have any good hypothesis on why they would do this, as Epic wants to run their own store and be the one earning commissions.
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.