Guys Exum isn't unfit, he's been at the AIS for a number of years, they have training every day and have their diets maintained from a young age.
He's gassed because it's his second game in 12 months, like all sports, it doesn't matter how much training you do, nothing substitutes for games, and it will take him most of summer league to get his legs back.
The other thing is the altitude. It's hard to understand, but Australia is one of the flattest and lowest sealevel countries on earth, we are an island, and despite you guys not thinking like it would effect him that much, that thinking is way off the mark.
My Australian Rules football side does pre-season training at the US Ski/Snowboard center at Park City, these guys are some of the fittest ball sports athletes in the world, they would absolutely blow a basketballer off the park without even trying in endurance testing, they run 13+ miles a game on a 185 yard oval field.
These guys absolutely struggle for the first week and half of training, and as I said, they are on a completely different level fitness wise. Their split times for most of their runs are well down. (I realise Park City is a far cry from Las Vegas, but it is relevant).
It will take his body a few weeks of getting used to playing in area's like Salt Lake City and Vegas.