How does ice sharpen kitchen disposal blades even for a short time?

Answer 1

It doesnt actually sharpen them, but it does clean them. A clean blade will cut better than one covered with gunk.

Answer 2

I wasn't aware it did but I suppose it's possible, depending on the hardness (MOH scale) of the blades, the blade design, angle of the blade edge, speed and direction of rotation. Ice, being releatively hard, could act as a sharpening device in the same way as traditional materials like abrasives by passing across the blade(s) in the manner that a sharpening stone or grinding wheel passes across a knife edge. In this way the same energy that dulls a blade might be used to sharpen it.
The previous answer is partly correct too, in that the ice would clean off most debris. But if the blade design, etc. was not correct, it would also accelerate the dulling process.