I am told this is the regulation being enforced in this situation. Exception only for Cup winner, but doesn't extend to Cup runner-up.
I'm also told that had Heidelberg won the Australia Cup, assuming they met club licensing criteria, they would've taken their place.
Football Australia has confirmed the AFC ruled Aus Cup runner-up Heidelberg United ineligible for the direct slot in next season’s ACL2.
Melbourne Victory take the slot as highest ranked Aus team on A-League Men table behind Newcastle (ACL Elite) Adelaide (ACL Elite playoffs)