Who said that land bridge and island hopping were the same thing?
Prior to the full exposure of the land that we call Beringia, the gradual process of water levels becoming lower meant that some parts of what later became Beringia would be exposed before others due to the uneven ocean floor beneath the Bering Straight. Likewise, already existing islands like the Aleutian chain would have more of their perimeters exposed as dry land, making them larger with resulting less distance between them. That would make it possible for a marine adapted population to cross between the islands.
It's like the reverse of what we are seeing today with global warming and higher ocean levels. Oceanic islands are not disappearing all at once. It is happening gradually. The first areas of islands to be submerged by water are the lower, sandy shorelines that are vulnerable to erosion. Harder, rock based land at higher elevations remains above water. The result is that the island becomes smaller with more water distance between the island and other islands. At the same time, the other nearby islands experience the same decrease in size and even continental shorelines get eroded. All of this land loss creates greater distances between continents and islands offshore and between the islands themselves.
During the full exposure of the Beringian land mass, the cold Arctic Ocean water was confined to north of Beringia. People would have been able to follow the southern shore of Beringia to the Western coast of North America.
The Alaskan and Canadian glaciers were merged together into impenetrable miles thick layers of ice and snow. Nobody was traipsing over them until they melted enough to become passable, starting around 12,000 years ago.
And yet, we have dates of human presence in the Americas going back to 20,000 to 23,000 years ago. They couldn't have arrived by land over the glaciers. So either they arrived by boat, bypassing the glaciers, or the timeline of glaciation has to be altered.
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