Frankly the scale of pretty much anything involving that wall is inconsistent as all fuck, and out of all the shots we got out of it that one is the shot I'd trust the least of all.
I mean, look at this:
On a semi-related note, I've my doubts about catapults being a good choice from that far up, even with the added ease of boats being a large, slow moving target, but I suppose it's not entirely nonsensical: maybe scattershot would be effective at countering the inaccuracies of a high arcing weapon starting it's arc from so much higher, the extra height and potential energy countering the lessened kinetic energy from lower mass.
Or maybe it's magically guided rocks that they're lobbing around.