part of speech - 'Really!' Is it still an adverb?

It is supposed to be an interjection, just like "Indeed!" or "You're kidding!" It can still be viewed is an adverb ("Is that really so!"), but that would be a stretch, since it can also stand in for "I can't believe some people still think that Hals and Rembrandt knew each other personally," which is a complete sentence full of various parts of speech.