They're a bunch of young players who got a year older and better. The jumps they've made are similar to the ones the Thunder made under Brooks, and no one is claiming he's any kind of great coahc.
It's one thing to jump from young lottery team to playoff team, but going from 51 wins (lower-rung, non-contending playoff team) to 67 wins (historically great) is much more difficult.
The Warriors didn't go from being a mediocre offensive team a year ago to elite this year simply by way of young players developing. There is a night and day difference in the amount of ball movement and motion in Kerr's offense versus Mark Jackson's iso-heavy offense.
It's quite probable that Golden State would have been better this year than they were last year if Jackson were still the coach, but they wouldn't have won 67 games with Jackson insisting on David Lee starting and playing 50% more minutes than Draymond Green. Lee wasn't even in the playoff rotation until Speights went down. Does anyone honestly believe that Jackson would've had the balls to bench Lee let alone, complete remove him from the rotation?
The Warriors wouldn't have won 67 games with Jackson's steadfast commitment to playing two traditional bigs at almost all times with a healthy Lee, with the vast majority of Green's minutes at SF. Believing that Jackson would have done things differently this year requires ignoring the fact that after the Warriors upset the Nuggets and pushed the Spurs two years ago behind a small-ball lineup in the wake of Lee's injury, Jackson went back to his old lineups.
The Warriors wouldn't have won 67 games with Jackson's hockey-style substitution pattern that resulted in several minutes of all-reserve units featuring a heavy dose of Harrison Barnes post-ups.
The Warriors wouldn't have won 67 games with Jarrett Jack running the crunch time offense instead of Curry, which Jackson chose to do two years ago.
If you want to question Kerr's coaching acumen by arguing that we don't know how much of the difference in this year's coaching versus last year's is due to Kerr having great assistants, then fine. But if you're arguing that there is no qualitative difference in Golden State's style of play and their efficiency on both ends of the floor beyond young players improving, then I don't know what to tell you.