You *can* open the map, the one that was originally generated (using its permalink). But you can't restore any changes you've made to it, such as customs names or added cities. It was originally (and still is) a generator, not an editor, so there's not much you can edit and saving those changes doesn't seem like super important. But over time, the amount of editable stuff is growing, so yeah, it will become possible to import back exported maps, probably soon.