Because he has the same type of eye markings that Naruto has in Sage Mode? Kabuto gets dark pigmentation around his eyes that also go down across his neck and down his back. It's just like Naruto's, albeit a little bit more excessive but it is there. His body looking all scaly has nothing to do with the sage mode. He looks like that because of Orochimaru. He isn't using sage mode ALL THE TIME, and his body obviously looked like that the entire time before he actually used sage mode in this chapter, so we can only assume that his body just looks like that and has nothing to do with sage mode.
Yes he gets horns also, but that could just be a part of dragon sage mode. There is no other transformation besides his eyeballs turning dark and his pupils becoming snake eyes, and the dark pigmentation around his eyes going down his back. So I would say that is a perfect transformation to me.
And if you are going to argue about the horns he grew, if he wasn't controlling the sage mode perfectly, I am sure there would be a hell of a lot more transformation other than just horns, look at Jiraiya and Naruto when he was training. Plus he did get the dark pigmentation around the eyes, which you only get when you PERFECT sage mode.