How would the imperium react to meeting another human galactic power?
How would the Isaac Arthur civ react to the imperium having such distant social values. Some feats of the Isaac Arthur civ:
Most of the shit the imperium does is actually legitimate survival strategies for their shithole of a verse.
How does the isaac arthur civilization deal with daemons and their mind affecting properties? How do they deal with psykers who open rifts to let daemons loose into their nice dyson spheres? How do they deal with machine spirits? (daemons born in the physical world inside machinery or artifice, and as such not corrupted by the warp initially and susceptible to becoming friendly to humanity via religious worship of the machine cult)
If the two civilizations meet in 40k's verse, then the isaac arthur civilization will immediately start breaking apart due to daemons and the warp. with segments of it surviving by converting to worship the emperor for the defenses it gives. The imperium would be bolstered by them, but chaos would be bolstered
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If the two civilizations meet in the real world. Then the 40k civ suddenly finds itself without FTL drives (since their FTL is based on travelling through literally hell, which does not exist in this scenario). And will not really be a threat to anyone.