Whilst I wouldn't have worded it as he did, you'd have to be really keen to split hairs to take issue with
Spymaster's description of the second limb of the definition of rape.
In practice, there's no real difference between saying that, to find him guilty they'd need to decide beyond reasonable doubt that he did not reasonably believe in consent, and that they'd find him guilty if they concluded he could not have reasonably believed in consent. The 'could not' is just another way of saying that they must be sure he did not believe in consent.
At most, you're arguing about whether or not 'could not' its an absolute, or implies some threshold to exclude fanciful possibilities. Which, given the context his position is pretty clear.
You're both coming across as a bit silly with this spat, on this thread in particular. Especially in light of Trashpony having shared what happened to her.