Originally Posted by Child-of-the-Emperor
Xenos were a threat to humanity during the Dark Age of Technology. The main difference here is that Humanity was no way near as Xenophobic as the Imperium is, thus (arguably) pointless wars weren't fought against Xenos simply because they were Xenos. In my opinion they were likely more 'passive' alien races than 'aggressive' ones in the galaxy, thus if it was to their mutual benefit non-aggression pacts were secured with many Xenos Species.
Aside from that the Eldar Empire was still present and the most dominant force in the galaxy throughout Mankind's Dark Age of Technology, and seeing as Humanity spread across the galaxy from Terra (As Human worlds were reconquered during the Great Crusade in every corner of the galaxy), its unclear what kind of official or unofficial relationship the Human Federations had with the Eldar Empire. But I think it goes without saying that the Eldar Empire was more powerful than Mankind was, even during the Dark Age of Technology - Thus Xenos did pose a great threat to Mankind, it just seems to have been 'diverted' given the many non-aggression pacts with many Alien races and the seemingly passive nature of the Eldar Empire towards Humans.
Yes, technology was obviously way more advanced back then, but I think it has to be taken into account that the Imperium is a military undertaking, the entire organisation is geared for war and war is all that it knows. The Human Federations however were not solely geared for war and were not properly unified like the Imperium is under Terra.
The Adeptus Astartes are the pinnacle of Mankind's Military Technology, created solely for war, never has the galaxy seen such a breed of warrior. The Human Federations didn't have such 'super-soldiers', and certainly not the same psychology - that which was of war, and war only.
I imagine that any hypothetical confrontation between Dark Age humans and Astartes would depend almost fully on circumstance, and thus it is extremley hard to give a conclusive answer.
All good points, although are we certain that there was no super-humans present during the Age of Tech? After all like you say they were more open minded and liberal than the Imperium so is it not likely that genetic technology was at least existent and likely fairly advanced, after all in the present day we're already well on the road to serious genetic manipulation.
Don't the Innterex have their own brand of genetic warriors, can't quite remember.
I'm not saying the Astartes wouldent be a potent and grave threat to them in a crude way.