Lamont v Postmaster General was decided the way it was because it required Dr. Lamont to make a positive and OFFICIAL act in order to receive something through a U.S. Government service.
The Protecting Americans from Foreign Adversary Controlled Applications Act requires no official (meaning related to Government) act on the part of the user. A secondary, but still important, consideration for SCOTUS in that case was that the U.S. Mail was an official Government body, that also doesn’t apply.
The Protecting Americans from Foreign Adversary Controlled Applications Act may still be struck down but Lamont v Postmaster General is IMO a poor case to use for comparison.