So, the idea of hybrid civs is that it combines the features of a full civ and a variant civ. More on it can be found on this Reddit page about this proposed concept: The next DLC will introduce HYBRID CIVS into the game : r/aoe4
Now initially, I didn’t think this civ kind would be a thing and work out, but after the addition of the Jin Dynasty, which seems to resemble a full and variant civ, maybe the hybrid civ classification may have to be added to avoid confusions between whether a certain civ like the Jin Dynasty is a full civ or a variant civ.