Settlers Took a 20,000 Year Rest in Beringia?

According to the new theory, humans heading east after leaving Asia about 40,000 years ago were blocked by two huge glaciers that met at present-day Alaska.

With no way forward, the humans settled on the land bridge, called Beringia, that connected Asia and North America.

There they remained for 20,000 years. Beringia was cold and harsh, comparable to winters in modern-day Siberia. Small populations of mammoth, bison, caribou, and other animals provided sustenance for the migrants.

It's a theory designed to make sense of some DNA research. In my mind it has a serious flaw - rather than spend 20,000 years in a cold, harsh place - why didn't they just go back the way they came?