My theory is this:
Kull Era Picts: Barbarous people, roughly around the level of Cimmerians developmental wise.
1st Cataclysm:
Conan Era Picts: Regressed from the cataclysm, losing alot of whatever development they gained before the previous cataclysm. Regress from barbarism into outright savagery, not too far above stone age.
2nd Cataclysm:
The Thurian continent breaks up, and the Picts find themselves on the British isles and regress further because of cataclysm. Peoples from Asia migrate to British isles, and drive the native Picts away from habitable areas into caves near swamps and moors. Inbreeding, worship of dark gods, black magic use and constant fighting from newcomer peoples drive them underground and degenerates them further into subhuman, dwarfish race. The race that drives them underground either takes the name of Picts, or is confused with the original Picts, and they are now known as Picts, but in fact have nothing to do with the original race. As time goes by, they keep the name, forgetting how they got it.
Time of "Worms of the Earth":
Roman invasion begins, the original Picts, now almost extinct with only a single colony left in the bowels of the earth, are almost a legend. Probably went extinct before middle ages.
So, thats my theory. Fantastic, my yes, but really, Howard's works have almost nothing to do with legitimate science. And keep in mind, the worship of dark gods and use of black magic was, it appears, in Howard's mind something that affected whether a race flourished or degenerated.
Just my thoughts.