Senjutsu is still senjutsu. It’s his own and he has enough of a mastery over it to turn into the hydra which uses the white snake sage’s power to transform.
How is it fanfic.
Orochimaru literally said “My senjutsu chakra was poured into this curse mark”
Meaning he has senjutsu and can control it
He’s a sage, albeit imperfect
So I did some digging and found out that the answer to this dusty old debate has been right under our noses the entire time. Orochimaru himself told us years ago that he himself is a sage, I just can’t believe no one ever noticed.