As pointed out on the forums[1] by the user known as Player of 0AD: mercenary units are trained as advanced rank in-game but are depicted as basic rank within the Structure Tree.
This is because mercenary units are specified as basic ranked within the templates of the entities that train them, and an auto-researched technology "promotes" them to advanced before a game session begins.
In-game, this is taken care of within the ProductionQueue simulation component's CalculateEntitiesMap() method.
This revision updates the Reference Suite common code to do the same where needed, so as to permit mercenary units to be depicted at the rank that players see at the start of a game session.