The falx had its AP removed so as to underpower the units that use it. Any falx-wielding units that still have AP simply mean there are inconsistencies in the EDU and that gg2 has yet to remove AP from those units. It's simply a move that was taken to underpower them. I could argue that AP be removed from maces and axes, especially axes. But when we look at non-blade weaponry, we see why AP is necessary. The falx is a blade, and this is one of the biggest obstacles in understanding it as AP-worthy, but if you consider how devastating the falx was, it doesn't matter if it didn't cut through armor, it still deserves AP as a way of representing its devastating nature as a weapon of war.

TL;DR Falx translated into RTW terms should be AP. The EB team, though not flawless, had this point right the first time around.