The so-called rising fastball is a myth. A pitch that seems to be rising is really falling at a much slower rate than normal, and the angle of the camera (higher than the pitcher) is what gives the illusion of upward movement.
THe camera angle also makes a left hander's curve look like it has more break than a right hander's, but not true.
Well that is what I thought too, and I was confused about it for a long time with Clippard (as well as others with negative pitchFX data). But after analyzing the data, I actually do believe Clippard's pitch has some rise to it (though I will admit it is very tough to verify). Not the so called negative rise that is very common in many four-seamers.
But you are correct, all pitchFX data is categorized as negative or positive. That data which is negative usually means exactly what you have stated.
Regardless of the semantics, it is a beauty of a pitch that always ends up very high in the strikezone.