Earlier quoted context omitted.
While the administration could say that Apple only has to block wechat from the US app store, it's not what they have done and they have not indicated that they want to go that way; the currently announced prohibition, as far as I understand, would prohibit Apple to support WeChat anywhere no matter if it allows installing WeChat for a US phone this way or not.
The administration doesn’t have the constitutional authority to make such a demand.
International Emergency Economic Powers Act (50 U.S.C. 1701 et seq.) (IEEPA)
the National Emergencies Act (50 U.S.C. 1601 et seq.)
section 301 of title 3, United States Code