Originally malt meant barley as wheat malt and beer was the preserve of the Bavarian nobility, the Reinheitsgebot was to protect the nobility monopoly on malt and prevent other adjutants been used. Yeast was only added centuries after and malt is generally defined as any malted grain ie rye, wheat and others
Maybe they define an adjunct as any malt that can be used solely as base malt, but you in theory make a beer from 100% aromatic, sorghum Rye malt. But I would have though that any malted grain has enough diastatic power to convert its self?