Truth and true are subtly different in that true, in this sense, is more about accuracy and facts. Truth extends into a person's veracity, belief system, and perception of reality. Truth tends to be more a collective term - "Your truth and mine differ" is a reasonable statement. I agree that "Truth" would be a less likely answer to the cited statement, but it might be an answer to someone saying "God created the Universe". "(That's) True" as an answer would discount a knowledge that that might not be universally accepted, while "(You speak the) Truth" would indicate that it matches the speakers view of reality.