I highly doubt anyone claims that Orochimaru has perfect sage mode, but he does have a good amount of natural energy, enough that we could classify it as imperfect.
For one, as you yourself pointed out, Kabuto is a perfect sage. When he first shows us he’s a perfect sage, he says quote “He [Orochimaru] could not become the perfect sage like I have!”
Now as basically knowledge on the English language dictates, specifying that he hasn’t achieved PERFECT sage mode by proxy implies he has a form of imperfect sage mode, because if he didn’t have that either, then Kabuto would simply say he couldn’t become a sage like him period. Specifying perfect would be redundant, and hints at this.
Further more we know that Orochimaru has to give the curse mark to others from his own body, meaning he actually has Juugo’s cells inside him constantly, and is just giving some to whoever he bites, which canonically means he is absorbing natural energy. The fact that he never has any rampages like Juugo’s clan suggests he can actually control the natural energy, which would be controlling natural energy, which means an aspect of sage mode, even if it’s not perfect.
Also, the art of the eight headed serpent jutsu is outright stated in the databooks to use the power of the great white snake sage to fuel the jutsu. The great white snake sage is the equivalent of the great toad elder, which would surpass Ma and Pa. So we factually know that his ultimate transformation is using a high form of senjutsu, but is obviously not at the level of Kabuto.
Also, Kabuto says he has become a dragon because Sage mode as made him exceed his status as a snake. So we know to go from a snake to a dragon requires natural energy. The art of the eight headed serpent is stated to be “Like a dragon god, having surpassed even Orochimaru”
So while he certainly doesn’t have perfect sage mode, we know he employs natural energy and controls it to a degree in his base form, and canonically utilizes natural energy on a large scale with his ultimate technique.
