Partially agree. There are not many instances of 'Barbarian Armies' in the Ancient Era: perhaps the 'Sea Peoples' that hit the Levant and Egypt, but most of the Barbarian Army effects are the product of what in Civ would be, well, Civs: the big wars were between groups like Egypt, Hittites, Assyrians, Babylonians - all Civs, or Hsung-Nu and China - mostly raids on either side, but not mass sacking of cities and abandonment of countless acres of territory.
On the other hand, the 'nuisance value' of hill tribes or pastoral mounted raiders was considerable, and if somewhat overblown, Civ's Barbarians represent that.
My view in Humankind is that the animals in the Neolithic represent the 'human nuisances': beating them up can be lucrative, and they can harass the heck out of your units (I learned the hard way that a Mammoth or Bear can beat the snot out of a scout or tribe unit easily) but you won't find a Mammoth charging up the steps of your palace, so they aren't really dangerous.
They could replace them with 'barbarian' graphics, but then they would have both 'random barbarians' and Minor Factions, which currently seem to fill the niche occupied by both City States and Barbarians in Civ, so in a way that would be redundant: better to leave them as separate graphically, even if the idea of Babylonians hunting down Mammoths or giant Cave Bears is historically dubious to say the least.