16 bit, I'm pretty sure. Also Wilbur confirms max value is "65535.0000" and lowest is "-0.0000".
PSP likes the histogram - so do I. It just doesn't "look" like there are 65K steps in height over that small distance.
It's one reason why I fancy using map tiles to "paint" the thing, the "grass" or "grit" or whatever masks the artefacts a bit too.