The idea of "other realm" is practically universal, it's shared by religions and psychology. It may very well be innate. Of course the atheists back then weren't like the atheists now, more of the "our images of the gods are just images we made and who knows how they actually look like" kind, presumably because science wasn't advanced enough for people to bet on materialism, as well as the gods back then still very much functioning as Archetypes not caught up in "can god create a stone too heavy for him to lift" type of bullshit. I mean we're a (mostly) serially monogamous species, of course Juno exists, duh, she's the goddess of that how can you deny the existence of marriage: Juno is the self-portrait of a specific instinct of ours (and yes of course she's married to sky-daddy (Jupiter)). And the gods are our ancestors because those instincts are from the genome... brought to you by The Ancestors which once were revered collectively before people learned to better distinguish different instincts. The structural similarity between paganism and the modern narrative is striking, isn't it?

Circling around: If you read De Ira what should jump out at you is that Seneca is spot-on about just about everything in there. Certainly better than the majority of contemporary self-help authors: The differing framework didn't hinder him, it's ultimately a detail that doesn't matter given the subject matter. Unlike middle-age authors, he doesn't get lost in arcane demonology (exceedingly fuzzy portraits of maladaptive complexes) but lays it out plainly. As such I'm more inclined to attribute to Freud the re-popularisation of a thing that was lost (the other-realm filled with archetypes), in a different dress, after the double-whammy of monotheism just doesn't making a lick of archetypal sense (there's structural psychological sense, though) and progressing science and materialist attitudes killing off its credibility for good. That is, yes, psychology is our new religion we just don't call it that and many like to deny it. How can you even be a Christian, with a personal god, if your mind also has the idea of subconsciousness firmly embedded in it? Subconsciousness evicted Jesus, now people often consider him to be everywhere or outside of physical reality or whatnot but not with them.