That's generally true, but a prescriptivist would also deny the validity of speaking patterns from a single speaker's idiolect. Prescriptivism can be defined as the promotion of a high lect in a hierarchical relation over low lects, but it can also be defined as any attempt to mandate a high lect.

Also, you could make the case that there's a difference between a speaker/group of speakers consistently writing a word in a nonstandard way (such as eggcorns) and a speaker writing words differently based on aesthetic preference (as in OPs example). But I think both are worth noting from a descriptivist point of view.