That’s mostly because of the outdated IAU definition of the boundary between those genres being at deuterium fusion. Deuterium has a low abundance, and its fusion happens very briefly and early in the lifecycle of these larger dwarfs. That flash is not significant in the grand scheme of what these objects fundamentally are, is highly theoretical and has never been observed, and is more a reflection of cold-war era fusion chauvinism than an actual morphological boundary between object classes.