I'm slow to actively call this a clone. It's possible that we are seeing a new genre happen here, much like Devil May Cry and God of War pioneered the 3D Actiony Quicktimey Adventurey genre. After all, people are still calling games like Castlevania: Lords of Darkness and Star Wars: Force Unleashed God of War clones, even though the genre is fairly well-established (and Devil May Cry predates God of War, but anyways).
That aside, I suspect that the main purpose of Fortress Craft is similar to that of Transmorphers
: feed off of people's desire to get something that isn't out yet, making a quick buck in the interim. The name is certainly... original enough (it's be like making a 3D Action game about a fallen god who battles and call it God of Battle). The graphics are much nicer, clearly; I've often wondered if Notch would be better off updating the graphics and lighting a bit (there seems to be as many that want that as don't, and at least a few potentials I know are disinterested mainly due to the graphics).
On a nerd-ragier front, some people are quick to say that Minecraft is just a rip-off of Infiniminer, a game that was worked on for a few months then abandoned and made open source. I don't think "ripoff" is accurate so much as "successor." But therein likes the issue: Fortress Craft is (likely) also based on the open-source Infiniminer stuff, and therefore as immune to criticism as Minecraft.
My judgment: I don't know. Fortress Craft, itself, is pretty suspicious (the bookcases look almost identical, and they have a big fat disclaimer that is usually reserved for shameless clones). But, they say imitation is the sincerest form of flattery, and it's possible that we've just got a new genre going here.
It's a secret to everybody.