As a person who subscribed (note: not mine, the original theorist is someone in this thread whose name escapes me) to the
"Eternum is the nexus point/gateway to the Multiverse/Astral Sea" theory after evidence and hints started showing up and pointing to that direction (though not fully concrete enough as of yet to make it undebatable fact), I'd probably pull an "Ashley Johnson after Sam Riegel pulled a clutch counterspell in the Final Battle of Critical Role Campaign 1" react:
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But yeah, as the others have said if you're coming into this from OIAL then there's already hints in the game that this takes place in the same setting, which would also imply that certain things that were revealed in the former are considered true in this game (i.e.
magic exists in the real world,
demons like Astaroth are from another plane of existence - likely natives from Eternum).
If you haven't played OIAL, then the reveal (assuming the story really goes in that direction and not be railroaded by a surprise plot twist) might come off as a surprise unless you're really paying attention to the hints and clues on the matter. And personally, with the way how the story is told I'm not sure if you could call it a "genre shift" given how the tech is never completely explained (which isn't the case in sci-fi stories where elaborate explanations on how tech in the world works are readily available) and is part of the game's overall mystery (which is what this game's "genre" really is - a straight-up mystery story with adventure elements - when you compare it to OIAL which plays out more like a Goonies/Stranger Things-style adventure-horror story).