The full film is on You Tube somewhere and shows more detail of the set up, calibre, dialling in details etc. I seem to recall from the original that is is easier to see that the moose flips back, gives a couple of kicks and is clearly stone dead. The guy comes across as knowing exactly what he's about and not as your usual 'yee hah' hero. IIRC, he is using some kind of jet boat
I'm not commenting on ethics but as long as you aren't calling it stalking (or even hunting) and you have the considerable skills to pull off a shot like that humanely... Then again, if he'd shot it in the arse and it had buggered off, I don't supposed he'd have posted the film on the internet.
A.