Mm. I can't use the code on my American region game. Also, I did specify most, not all, hacked Pokemon will have perfect IVs. Considering there is s 1 in 46,656 chance a legendary will have perfect IVs, if someone put the effort in, I doubt they would trade it away.
On the contrary, though, if it has reasonable IVs, it is probably legitimate. If you traded it via GTS, it is at least a good enough fake to go undetected. I'm guessing this Pokemon is legitimate, though.