It's both though. You need a QB like Mahomes to really succeed with Reid's scheme. He had all the pieces in Philly but couldn't quite seal the deal. With even more talent and a QB like Mahomes, he's getting even better results. So it really goes hand in hand. Belichick needed Brady to have the success he did. When Brady left, the Patriots pretty much fell apart. If Reid goes anywhere else, or Mahomes goes anywhere else, Reid isn't likely to be able to continue have the level of success he's had with Mahomes.
And I'll be clear, Reid is an excellent coach. Absolutely, unquestionably.
Sometimes a coach and QB compliment each other very well. Same with Payton and Brees.
You can't have one without the other.