The wonder card format is the same between SMUSUM games. You can illegally drag the wonder card into USUM, but there's no legitimate way to obtain one from the demo. (and the mon can also be traded inbetween)
Some people try to redeem gifts in illegal games, then make the adjustments themselves.
Or maybe they're fine with illegal received game.
PKHeX cannot predict what the user wants. So it just warns them that it is incorrect. It is up to the user to follow it or ignore it.
If you want it correct, either import it into the correct game, or correct it yourself.