I don't think you can say it was just a superior engine, it wasn't exactly a war and Bellew was spent by about round 7 and he was probably in the best shape of his life (although 36yrs old). That was mental and physical pressure from Usyk. The way he fought was probably the only way he had a chance, or just going all out in the first 3 rounds. Either way he gave credit where it was due and did as well as he could, for that he has my respect.
I think fitness was the main factor in the win for usyk. the first 5 rounds it was pretty close, some good movent, counter punching and tactics from Bellew. After that he slowed down badly not because it was a war but because of the high concentration and continually trying to fight off the backfoot is draining especially when he was loading up on his counters. Soon as he started to gas it meant he meant he couldn't implement those tactics successfully anymore and thus git sloppy and left him self open to the k.o I'm not saying usyk isn't a superior fighter but that Bellew didn't embarrass himself in there with his boxing, at times he out boxed usyk and certainly can be proud of his performance. But ultimately I think it was a superior engine that won the fight because Bellew gassed and usyk didn't. If it was six rounder it would have been very close indeed.
If it was six round fights Usyk would also start faster. He lost the early rounds when he fought Hunter and Breidis also. Bellew did as well as he could and for that he is to be aplauded, lost to the better fighter on the night that's all.