I don't get your point? Does that mean that anyone that uses senjutsu is a sage? If so, then I have no qualms of that. A bit dumb though since that makes everyone even Mitsuki a sage.
If not, then I disagree. He has to have his own affinity to be a sage and since he contracted and learnt from the snake sage, it means that he cannot be the sage. The best candidate is Kabuuto (I don't think there was a dragon sage and he created it by perfecting/tweaking Orochimaru's snake sage if I remember correctly).
As far as we are concerned, the sages are tree, aliens (6 paths), snake, dragon, slug, toad.