Yeah, for as many fans as have come in through one of the adaptations--generally the stage musical or its 2012 film--there's surprisingly little fic that's purely adaptation-based; there's not a firm line between brickverse and musicalverse canon. Probably the majority of people making fanworks read the Brick eventually, but even the ones who don't absorb a lot of it through fandom and through fanon.

(And there's a huge number of modern AUs--and other historical AUs, and fantasy AUs, and crossover AUs--where it's very difficult to say exactly what the source canon is.)

That's part of why writers persistently tag multiple versions of the canon or "All Media Types"--to acknowledge multiple layers of influence.