Aztec script is a bit more complicated than simply ideographic. There's a whole bunch of syllabaric stuff we don't fully understand as well. Regardless, defining "prehistoric" around the presence of so-called complete writing (scripts that can represent any spoken statement) leads to a lot of really silly situations, especially in the new world. For instance, did the Aztecs suddenly become historic as soon as the Spanish showed up? If so, why didn't the Mayan script (which is also complete) count? If it's that the Aztecs who couldn't write simply weren't historic yet, does that suggest the vast majority of Europeans who also couldn't write were prehistoric?

Either way, it's a terrible definition. As an archaeologist, I use "prehistoric" to describe really old fossilized crap. Anything newer is historic.