I don't know how much I really believe in this "QB Guru" stuff. There's part of me that believes a QB either has it or doesn't. It's one thing when these guys are in high school, but it's another when they are already pros.
That's kind of like saying post-college track athletes either have it or don't - and don't need coaches anymore.
Lots of coaches - Bill Walsh, for example - are/were really big on coaching technique at the pro level. No college athlete is a perfectly polished gem when they enter the NFL.
I do think, though, there's a certain amount of innate accuracy that can't be changed. Some of accuracy is purely genetic. But completion rates go up when QBs improve their footwork, throwing mechanics, ability to read defenses, progressions, pocket presence, etc. This is where QB gurus can help.