Bummer, eh? But it is still important to keep sight of the fact that the GAME is NOT OVER. It might take a LOT of fish introduced into Lake Michigan to result in an established population. And it is possible that even a very large number of fish introduced would not reproduce there, although I think that is unlikely. Mainly, keeping the number of fish that get through into the Great Lakes is still very important. It is not time to give up and drop the barrier.
The most likely explanation of the finding of eDNA is a fish out there - but there is still a possibility that it is not a live fish. Any number of crackpots out there that could dump a carcass and cause this hullabaloo.