I understand that, in just saying it takes energy. Not every UFC fighter can move like that for even 3 rounds without tiring, granted there is a lot of other activities going on.
I also believe not every fighter has insane levels of fitness.
I think a decent example is Conor. One post fight breakdown I watched on the Diaz fight specifically talked about Conor's slips as one of the contributing factors to him tiring fast. Conor slipping most strikes rather than blocking or parrying was said to spend a lot more energy.