Either the descendants of the Windover population died out entirely, leaving no genetic legacy in any surviving population.

Or the Windover population was subject to a genetic bottleneck — a dramatic reduction in population size — at some later point, after which the specific genetic markers characteristic of the Windover group were either lost through chance or diluted to the point of undetectability by subsequent mixing with other populations.

This is not a statement that the Windover people have no modern descendants.

It is a statement about the specific genetic markers available to analysis, and about the difficulty of tracing genetic lineages over 8,000 years of subsequent population history in a region that experienced significant demographic change over that time — including the catastrophic depopulation caused by European contact and colonization.