I think the extra "protein" helps for sure. This can put a lifter on par with someone else with superior genetics. But your brother is like my first example. With great genetics, "protein", or both, eventually you hit plateaus with random programming. Had he been training better, learning what really works for him, etc. he probably could have lifted even more. I know one super elite lifter on protein that worked his way up to a low 900's squat. He was following programming, but by experimenting with different programming and changing some aspects of his form, he was able to add over 100# to that lift. And this is someone with over 15 years of lifting experience.
It's hard to put percentages on it, because everyone is different. This can even mean how well people respond to "protein." Like the Dan Green example, he added over 100# to his conventional deadlift pretty late in his training career by changing his form and programming for that lift. There are also pretty heavily drug tested lifters that out total guys taking lots of stuff. They are probably clean, and even if they aren't, they are very limited to what they can take for compounds and dosages. Ray Williams in the USAPL (drug tested, even outside of meets) squats more than any non-tested lifter competing raw. So genetics plays a huge in role in conjunction with good programming, which you figure out over time as an advanced lifter.
Perhaps the better question is, what does the training HISTORY of these elite level athletes look like? At the advanced level, their programming will not resemble what us mere mortals do. But those people didn’t start at the advanced level...they started like all other schmucks (albeit probably naturally stronger). So the question you should be asking is, how did the elite lifters progress from novice to intermediate to advanced to world stage? If you investigate the training history, I suspect you’ll find a lot of stuff that looks very similar to SSLP and PP intermediate templates, which those lifters used until they eventually stopped working.