"Truth" as a reply instead of "True"

"True" is short for "that's true", and is definitely more widespread than "truth". "Truth" will definitely sound like slang to others.

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.