True. Kenpachi can beat Nnoitra, number 5 Espada, and Grimmjow is only 6.
If you remember the fight between Kenpachi and Ichigo, Kenpachi was fucking destroying Ichigo, easily. Ichigo got back up and had a surge of fighting spirit. Kenpachi was so impressed he went for a one-blow showdown. This was the worst thing he could have done, since Ichigo has a crazy amount of Reiatsu and a one-blow showdown ins where he really shines.
Even after that showdown, though Kenpachi said Ichigo won, Ichigo definitely came off worse damage wise. He would have been cut clean in two if not for his Hollow-mask.