As far as the gesture analysis function is concerned a swipe is fundamentally a drag gesture that's moving in one of the cardinal directions, which is to say swipes are a subset of drag gestures. The speed of the fingers or the time for which they contact the touch screen isn't considered, so a quick temp swipe on the screen and a slower, well defined movement should both be detected as a drag, and depending on the direction of movement also a swipe. It is for the reason the Gestures GUI has a setting for choosing between the two.
What's going on behind the scenes in Gestures is that when the analysis determines that the touches are performing some manner of drag gesture, it also reports the swipe direction, if one is detected. This way an app using the analysis can decide which specific gesture it wants to respond to.
We designed the analysis to be more general purpose, reporting all of the relevant information about a current gesture. There's separate logic in gestures to take the results of the analysis and decide which of the specific gestures in the GUI to respond to (like swipes vs. drags) based on the current gesture settings.
Note: As of version 2.0.32 we both fixed a bug where scrolling using slow moving gestures would produce jerky, undesirable motion and generally improved feel and smoothness of scrolling and scrolling momentum so that it is now more similar to using an Apple trackpad.