The entire premise of the NFL contradicts what he's saying.
It's not just OL and QB either. There are definitive body types and skill sets that all NFL coaching staffs look for in players.
If every coaching staff wants a definitive body type and skill set, it totally supports what he's saying.
Since we regularly take great athletes and throw them into positions they never played in high school, scheme itself wouldn't appear to be a major issue.
Except for quarterback. Like everyone says.
Except, you can't have a 250 pound DE in a 3-4. Let me re-phrase, you can have 250 pound DEs in a 3-4 if you want to get dominated along the line of scrimmage.
Basically, finding players who can physically do what you're asking is what it is all about (contrary to popular belief it is NOT the hokey pokey).
That's "finding players that fit your scheme."
Not all LBs have equal coverage ability. Not all LBs can rush the passer equally, etc.
Maybe we're discussing semantics here, but I see what goes on in the NFL as coaches and GM's using the draft and free agency to find guys that "fit" what they want to do. You can call it scheme, you can call it "fit", call it anything except late for dinner...but "fit" is huge.