She is not evidence that Neanderthals and Denisovans were constantly interbreeding throughout their range.

She is evidence that such interbreeding happened, that it was physically possible, that the offspring were viable, and that a specific encounter between specific individuals was biologically successful enough to produce a child who survived for years before she died and was buried by the cave.

How many other children like Denny existed — other first-generation hybrids who lived and died in the ancient human world without leaving recoverable genetic evidence — cannot be determined.

Some fraction of them presumably survived to reproduce, and their descendants are the people who carry the signal of cross-group ancestry in their genomes today.