So with gersons it looks like monsters telling of the war was wrong. But that implies some questionable stuff:

either

Toriel and asgore allowed monsters to slowly forget about how the war was actually like. Toriel and asgore both being neglectful enough to not notice for thousands of years. Or at least not caring enough to stop the simplification of history.

which is odd since would toriel and asgore really not care or notice. Sure toriel has some neglectful issues and asgore is asgore, but this would require them not noticing for around 1000 years

or

Toriel and asgore did try to stop it but once she left, asgore started modifying history. Making the monster population accept the idea of the seven soul plan

which is odd because I dont think asgore would do this. Asgore doesnt seem to be the type of person to do this. Also he doesnt want the genocide plan to happen, in the first place

or

Same thing as before but asgore just let people forget.

Issue is 100 years isnt long enough for people to simplify and forget stuff. Also the time when toriel left, is the time monsters had modern technology, and modern society shit. There would be modern archival methods that would have prevented monsters from forgetting, easily.

or

toby didnt think about the implications of what he wrote here.