[advisedwang]

My lay understanding is that the theory is that these earlier humans crossed the Bering land bridge but that they didn't proceed through a inland ice-free corridor (as there was none then). They likely traversed the pacific coast, which was not at the same position it is today and so would leave no evidence.

I have also seen discussion of sea arrivals, but they are much more hypothetical.

Also worth noting as the earlier arrival doesn't rule out the later ice-free corridor traversal as having also happens - it just means it wasn't the first people to arrive in the Americas.