User blog comment:Kuopiofi/Saitama - Absolute or Supernatural?/@comment-4714517-20160906191402/@comment-5265497-20160908005710

"He would simply destroy the planet and move on if he was an actual planet "

His final move was a planet buster (anime) or probably a life wiper (manga), not his casual attacking ability. He has to release all of his energy to execute that ability and he was just searching for an opponent that can fight him on equal terms. He's not going to use his trump card in the very beginning of the fight as you seem to assume.