Avant-garde does not precisely mean “unlistenable.” There’s plenty of music that’s listenable but still way “out there.”

Glass’s music is maybe not consistently on the edge, but his avant-garde reputation comes from works like the “opera” Einstein On the Beach, which is a formidable listening and performing experience, even if it’s heavily diatonic and based on familiar major/minor modalities. It is a sui generis approach to musical structure and musical theater.