Isn't that true by definition? SCOTUS is the final authority on what is constitutional, no?

It seems cleanest to see Constitutional as a two-place word, and to point out that the government's written policy is to accept CurrentSCOTUS!Constitutional as the binding word. (The SCOTUS can overrule a previous version of itself, for example, which means it's not quite final.) It's popular to describe SCOTUS decisions as "morally wrong," but more relevantly, it seems that they could make decisions that are "logically wrong" and thus aren't Constitutional in some other important sense.

There's also commentary here and there about what the Constitutional duties of the non-SCOTUS arms of the government are; the President does have, as part of his oath of office, defending the Constitution, which presumably could require him to stop an insane SCOTUS out to wreck everything, but mostly people discuss in context of presidents signing laws they believe to be unconstitutional.