He's obviously not weak. The BSM (not KCSM clearly since he let Kurama loose in it) Naruto of the Last most likely had some degree of Rikkudo chakra within him that enhanced the mode give his flying. However, even if it were simply a 100%BSM Naruto, the fact that Toneri can match the physical attributes of a BSM mode stronger than the Juubito fight one while unleashing absurd firepower makes him one of the strongest characters in the series. Using "a single punch" as proof that he's weak is cherry picking. Did one punch finish him? Yes. However, it was the strongest punch in the series. If the gigantic crater from the shockwave wasn't proof enough, focusing his chakra into his fist enabled Naruto to walk through Golden Wheel, which did cut the moon in half. But hey, leave it to haters to ignore scale and downplay a character.