Yeah, it's almost like the "Completed" tag has been added prematurely... A bit of common sense just needs to be applied, but from what people more in the know than I have suggested, there's no room for that sort of common sense in the forum rules. The epilogue is part of the story, so it's not complete until that has been added. The devs haven't helped themselves in that respect, by saying the "main story" is complete, or however they worded it, and pushing out the Steam release which apparently is why the "Completed" tag got added.
