I somehow doubt its a remaster because that would be quite expensive to do. A lot of the environments would need to be completely reworked, and lets not forget that Onimusha 1 - 3 uses the likenesses of real actors (one who has passed away).
Remasters would be cool though, for preservation purposes. But I think its a new game.
In fact, lets go a step into bizarro land: the medieval Deep Down has gotten a face lift into feudal Japan. Co-op Onimusha incoming!!!!
I suppose our close reference points are the RE0-1 remasters - seemed to turn out to be great games, but how about the dev costs? either way, the truly old assets have been reworked (or upscaled?) nicely.
The dead actor likeness however complicates things a little bit... but if it's a new game, more power to Capcom, provided it's not mobile