I don't think this is correct. As I read it, the issue was that, as others have said, the federal government attempted to regulate what laws a state could pass (which is related to a broader body of law about federal laws that
compel states* to enact certain laws or to implement federal laws). Note, however, that this not to say that congress can't make its own laws and that, if it does, they would override state laws.
Maybe one way of summing this up that makes more sense of the distinction is this: Congress can make laws that regulate behavior as a general matter (subject to other constitutional restrictions), but cannot regulate directly the behavior of state governments themselves.
It's unusual for congress to do the former but not the latter, but that appears to have been the case here.