This is not just telling someone there's a winning move. It's telling them that X is a winning move and they just have to verify that X is indeed one. Many, many problems are such that it's difficult to find a solution but easy to verify one.
I know nothing about this but I stay curious. Just like Navier-Stokes equations can be proved with numerical approximations, can this be verified even if we never solve it?
True, but in this instance it’s my understanding that there were still some areas of guesswork where the paper wasn’t clear.