Section 2.D.1 of rules (PDF p. 32) states that : Sub-generals can be allies with a dedicated command - the number and type of allies allowed for each army are shown in our army lists..
This has to be combined with the rule (non-contradictory, fortunately) at the beginning of each army book : [...]Unless specified in the notes, the general must be the same type as the army commander in their own army’s list but cannot be legendary. Usually this results in 1-3 UGs being compulsory and you having full flexibility on the rest. [...]
My read is that : the general commanding the ally, although with the same type as the Army Commander, is not an AC, but a Sub-General. Therefore he cannot command a unit reserved for AC.
Best,
Antoine