Knowing little about cpp modules and nothing about Gabriel Dos Reis, I expect a more design-by-committee type explanation for the result: the module system was probably a victim of having to be backwards compatible, abi stable, idiomatic, zero cost abstraction, be compatible with all weird cpp features, not hurt compile time, etc etc etc

I don't think its fair to attribute it to lack of skills or bad intent, unless there's some proof to any of it.