Keep in mind they've been going 110% in camp for days now and Woody doesn't give them much rest. Siler was clearly tired after a day or two in the previous camp (considering he was just coming off a active summer league), but when he was fresh early on, he impressed. If he only plays limited minutes the first couple of years, conditioning shouldn't be a problem.
Regarding his FT percentage, he shot a 66% average, despite his awkward mechanics. Not great, but not horrible for a big.