Why use G29 during a print?
Now, all of that aside, I guess, in theory, it WOULD be possible for the printer to use these 16 measurements to dynamically ADJUST the height DURING THE PRINT based on where the head is SUPPOSED to be versus where the ACTUAL height of that spot actually IS. However, I cannot find anything that claims this is what happens

This is actually what happens. This behavior is clearly specified in the firmware documentation:

Mesh leveling measures the height of the bed at several points to generate a 3D mesh representing bed imperfections. With compensation enabled, the nozzle follows the bed contours by dynamically adjusting the Z position (using bilinear interpolation between the measured points). The nozzle follows the contours of the bed more accurately with more mesh points and/or finer segmentation of moves.