Why would they have any idea what Terra used to be like? Well, I suppose it's already the case in story that they don't know that, given their opinions on earth. So that isn't really the relevant part to question. I guess what I'm trying to say is, why would they ever believe that? I'm saying that in spite of any amount of evidence that could be provided, someway or another.
I'll admit I'm only vaguely aware of 40K lore by means of osmosis, but I'm fairly certain it's the type of universe where if you suggest that Terra hasn't always been a shining beacon of ultimate purity (which it totally still is, not at all an ethically corrupt, overhyped tomb for a shiny skeleton) then you immediately get shot by a screaming testosterone man.
Earth Bet very much does not live up to that image. And it isn't even the earth that becomes Terra anyways, so their "incorrect" assumption about the planet they're on is in a way actually more correct than the alternative.