I think it's one of those canards that was repeated so often people just assumed it was a fact. I know by college (2001-2006) I already knew it couldn't be dinosaurs from biomass alone (but I was in Archaeology, so it's not a far leap to understand basic geology/paleontology). I suspect you're not alone and that a lot of people still assume dinosaurs.
I'll (somewhat shamefully) admit my guilt on this topic, but (in my defence) I didn't really put much thought into it.
But it's pretty obvious that it couldn't be dinosaurs unless they lived packed like sardines across hundreds of kms. Hell if you crushed everyone in New York into a cavern pressure cooker as tight as you could fit them and turned them into a goopy human stew, you'd end up with what, a couple thousand barrels by the time some matter broke down, decomposed, degassed and were generally squeezed by tectonics.
