BoE calls necromancers “wizards” and has a very rudimentary understanding of how it all works, anyway my point is for nona the ninth, i posit that “zombies” is referring to people ressurected by either necromancy a related thalergetic/thanergetic power. so either John’s ressurections (his powers dont seem to be garden variety necromancy) or revenent posession (like what wake did in cytherea, or what palamedes did with animating the hand bones that harrow made).

the summary says the zombies are “coming back”, which means it has happened in the past – we know John ressurected people at least once in the past and we know canonically in HtN had a whole bunch of ready-to-ressurect bodies just sitting on the Erebos.

we also know that something is Wrong In The River – souls are getting stuck, which could possibly make it easier for said souls to stay around as revenents and possess random corpses.