You're talking about two different things.

One is the legal side. Legally, you cannot go out in the street and punch the first child you see in the face.

The other side is purely practical. For sure you are capable of doing it, so if you also have the intention of doing it the only thing that remains to be seen is whether anyone is capable of stopping you.

Justice Sotomayor is not claiming that Trump cannot do this practically, she's merely saying that it would be illegal. And sure, it probably won't stop Trump from trying, but as a Justice of the SCOTUS, she's kinda supposed to comment on the legality of stuff.

As for pundits, they should have realized a long time ago that Trump didn't give much of a shit about the legality of things, and focused more on what he is practically capable of doing than what he's legally capable of.