For better or worse, he couldn't have actually done that; governor's powers within their own states are squarely 10th Amendment issues. Their abuse of emergency powers was not "illegal."

Trump did not, himself, declare a federal state of emergency. That's all he had the power to do (or not do). He cannot force state governors to stand down if they are acting lawfully within their state constitutions and are not violating federal law (which they didn't).