Well I mean that is what sources state, Kun was the most powerful Ancient Sith. His being > Nadd -whom shares Muur's tier- before his prime, is only greater indication. Plus Kun's implied living power compared to his spirit's is crazy enough in the first place. His spirit corrupting Kyp to on/beyond Vader tier is crazy, his spirit still being a lot more powerful than DS!Kyp even by that point is crazy.