Unfortunately, that’s not what “ex post facto” means.
Ex post facto involves making past conduct illegal, not future conduct.
Example: You eat a hamburger on October 1, 2020. Congress passes a law that says, “Anyone who ate a hamburger before October 2nd, is guilty of a felony.”
A ban on future conduct is not “ex post facto.” It may violate the 2nd Amendment, but it isn’t “ex post facto.”
If Congress banned all Pistol braces tomorrow, it wouldn’t be criminalizing the conduct that you already committed, but future conduct going foward.
It’s still unconstitutional, but not on ex post facto grounds.
The headline indicates 3M to 4M current gun owners could be felons.
That is wrong precisely because it is "past conduct".