Of course "build order wins" are a thing in every rating bracket, in fact I would say the impact of a build order is inversely proportional to your rating - that is to say learning a buildorder will have a larger impact on your performance on lower levels of play.
Btw nobody has to create buildorders, it's perfectly possible to improvise better buildorders than your competition's premade ones even at the highest level. You could just as well tell the high level players to "learn to play" as you may do to lower rating players.
It makes total sense that prepping would have a negative impact on many low level games. Like for instance a 1k player who has prepped 50% of the maps in the pool might be crushing half of his games while getting rekt in the other half. Ie. all of their games are unbalanced as the result.