For at its basic core, it being MGS5, I'm a little disappointed. Why hide that for so long? Ironically, maybe that was the surprise?
I feel like you could have easily guessed that was the case from previous trailers and the fact that it uses the Horizon engine.
Either way, the gameplay loop is most likely not the same. DS looks more like traversal/ parcel management/ stealth for the real world, with stealth/ action for the upside-down realm. So the latter is probably more MGS with the former being something a little different.