Adding natural energy isn't senjutsu, that's the reason Minato's rasengan could barely break the black shield, instead of getting rid of it as Gamakichi's Oil Gun.
Gamakichi didn't get rid of the black shield, Obito just lowered it. It was water not acid, the attack was completely blocked.
The point I'm making is that senjutsu, by definition, is spritual, physical, and natural energy. Chakra is spirtual and physical energy. So adding natural energy to chakra is the same. The fact it "barely" broke through omyoundon proves this, because if it wasn't senjutsu it would have been negated. H = S+P+N, C = S+P => H = C+N
Don't get me wrong, it's not on the level of what Naruto's senjutsu attacks, as his are in perfect balance and created internally, but it's enough to permeate Obito's defense.
Pa Toad proof, he specifically says Senjutsu is chakra augmented with natural energy:
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