>That doesn't really explain how they know to find this place, decades after the last time any member of their species visited it.

Because that's not what happened. These fish managed to get there because it was a good place for them to go, not because they were 'returning' to a place they had been before. The 'return' in the title is more about the fact that they are coming back to fill a niche in an area fish were blocked from, not that these specific fish were returning to a place they had been before. It almost seems like they were intentionally muddying the waters with the language used.