Chokepoint

Ineffable Island is looking at the Glacial Maximum's coasts. Those coasts were further out than they've been since the Neolithic; we had additional land area comparable to the area of South America today.

Most of that in Sundaland, I think; also, the Black Sea was cut off, so smaller, so delivering much more coast around that. Doggerland was there of course. I'd also thought there would be more land in Beringia between Siberia and Alaska, but that's not this paper's focus.

Before it got That Cold, it was still Pleistocene so frosty. Remember that we're in an Eemian interlude today. As of the 30s kBC, researchers believe that humans (and not Denisovans) had managed to reach the furthest Siberia. At 30 kBC, Beringia wasn't there yet - but a bunch more islands were. I'm guessing these islands were moraines deposited by an earlier Glacial Maximum.

The islands popped up from west to east, from Siberia to Alaska. When the first island was visible from Siberia, Siberian fishermen rafted over. Then they kept rafting over. Then they were on Alaska.

They don't talk of when full Beringia was exposed, but nobody is saying it wasn't. So I can guess. By 20k BC the Alaskan side of this ephemeral subcontinent already had a population sufficient to resist any further Siberians. These bottlenecked the coast and, north of that, the white bears did the rest. And southeast of that was blocked by ice and even bigger bears.