Turns out there's multiple threads discussing this one unit, so posting this idea in various places
If we're right about the speculation that the unit is indeed an Assyrian Siege Tower (or some other name), then it could 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.