The difference, and it's small yet big difference, is that if you are doing giant sets you aren't likely doing the first set to absolute-absolute failure. You go to approximate failure, or you appear to fail but you didn't quite give it 100%.
As an example. Many pros say they do so and so many sets to failure on legs, some say they do drop sets even, each drop to failure. I would say not quite.
Look at 1:10 with hack squats:
Do you think Chris would do his failure sets this hard at Gold's Venice? I doubt it.
You cannot do Milos style giant sets on legs at this intensity level. Especially not several rounds of them.
there is only one "failure", thats when you simply cant do another rep
How many bodybuilders actually go to failure?
Do a set until you get so you cant do another rep, thats failure, OK then drop the weight 20% and then carry on, see, failure is very difficult to get to.
Its like Tom Platz famous quote "when you think you have reached failure you still have another 5 reps left in you"
If that were true he would still be on his first set of squats.
Everyone training seriously pretty much does the same thing, and I have never seen Dorian push himself as hard as he pushed Chris in that video