The word you're looking for is "preempted", not "superseded".
What it means is that states can't write laws that contradict Federal law. It says nothing whatsoever about requiring states to enforce Federal laws; that's the Federal government's job.
There are exceptions where states and the Federal government exercise concurrent powers (e.g. taxation, creating courts, building roads), in these cases the Federal law may or may not preempt the state's, depending on the circumstances and the wording of the laws in question.
It's also the practice of Federal courts to avoid recognizing a new preemption unless the Federal law explicitly says so or it's clear that Congress intended to preempt.
But all of that's beside the point if we're talking about the President's recent threats to "defund" California.
The courts have made absolutely clear is that the Federal government can't withhold funding from a state as political punishment, particularly over something unrelated to the purpose of the funding. Congress (not the President) sets the standards that a state has to meet in order to qualify, and if a state qualifies then they're entitled.