This is right down my street, since I’m the premier expert on American law, and where its legitimacy fails.
To begin with, SC’s are mutually in error with the US government; since both err in their legal arguments. The US government claims to derive its authority from the Constitution and international law, while SC’s likewise claim that the US government holds no authority over them.
Both claims are false, since the supreme national authority in each state is the will of its respective People, as noted in the Constitution-- not as forming a single nation-state whose final authority rests with a republican government (since this would be not a democracy, but an oligarchy).
So only the CSA had the correct legal argument: i.e. that each state is a separate nation-state unto itself, but supremely ruled by its respective People (i.e. its citizen-voters). After all, that’s how we got the Constitution in the first place; and they didn’t alter that status.
So while SC’s are wrong, they’re not alone.