Going from 205 to 236 sounds plausible, though is a hefty amount for Cormier.
I weigh 188 lbs when I wake up on an empty stomach and then 193 at night after a whole day of eating; and I have a sedentary day job and only lift a few days a week. In terms of glycogen stores, I'm close to what the average person holds - 500 grams (1.1 lbs) in the liver and muscle tissue. So if I tried to nearly deplete my glycogen stores, that's a change of 6 lbs or so, without dehydration.
Cormier is larger than me and requires significantly more calories to maintain his weight. So food weight for him by evening likely adds a couple more lbs. Plus, with his size, and being a professional athlete, he's storing more glycogen.
I figure, with him, he added around 7 lbs or so of food weight and replenished about 1.25 lbs of glycogen between official weigh-ins and the next evening before the fight. So subtract 8 lbs from 31 and he's dehydrating 23 lbs which, if he walks near 240 lbs, is nearly 10% of his body weight in fluids. And at 10%, that's when one may experience kidney failure. And we all know he had a history of that.