Heinage
Khan
I'm leaning toward replacing the Chariot Archer myself. Assyria is already an Ancient Era warmonger. Giving them a siege replacement to spearmen would invite too much comparison to the Huns. They have to do something to differentiate them. Replacing the warrior and the archer is not as good, because you get warriors right off the bat and archers are necessary for defense.
If we're right about the speculation that the unit is indeed an Assyrian Siege Tower, then why can't it be an "early catapult" that they receive, much like the Mayans receive an "early archer"?
There's already a precedent of this happening with the Atlatlist being unlocked with Agriculture (first tech) and the Mayans not getting an Archer with Archery, so I think it's quite possible that the Assyrians unlock their Siege Tower at, for instance, Masonry, and then not get a catapult at Mathematics.
I reckon the the Siege Tower would then also be slightly more powerful than a catapult when attacking cities. Like maybe 150% strength vs. cities or something.