Statistically speaking, while we don't have the actual numbers for futa individuals (for good reason in lore), there are technically more futas in the game than there are women who are 6'5". We know of atleast three of them, two being alive still and we have yet to see anyone coming close to Jade's height, even amongst men. This again kind of furthers the idea of an initial impression leaning towards futa than not.
Moreover, this is what Maddison decides, what you and the rest of us don't know is if she has some fixation on futas. There very well could be a reason why that is her first idea other than Jade's stature, that being perhaps one of her relatives being a futa, or having a sexual or non-sexual fixation on futas.
I mean, this is a VERY strange hill to die upon, but you do you buddy.