It comes from 511.3 (C)

(C) Major Repair Garages. Where flammable liquids having a flash point below 38°C (100°F) such as gasoline, or gaseous fuels such as natural gas, hydrogen, or LPG, will not be dispensed, but repair activities that involve the transfer of such fluids or gases are performed, the classification rules in (1), (2), and (3) shall apply.
(1) Floor Areas.
(a) Ventilation Provided. The floor area shall be unclassified where there is mechanical ventilation providing a minimum of four air changes per hour or one cubic foot per minute of exchanged air for each square foot of floor area. Ventilation shall provide for air exchange across the entire floor area, and exhaust air shall be taken at a point within 0.3 m (12 in.) of the floor.
(b) Ventilation Not Provided. The entire floor area up to a level of 450 mm (18 in.) above the floor shall be classified as Class I, Division 2 if the ventilation does not comply with 511.3(C)(1)(a).

but similar language shows up in other places. That got rolled up into the IRC but it really refers to commercial garages.
The drill example is ridiculous because it has a 6 foot cord on it and it still can easily be run below 18", no matter how high a readily accessible receptacle is.
I don't know what the hell they are talking about with the cover since an explosion proof assembly is far more than just the cover. It still has to do with the idea that the area in a garage below 18" is a classified zone, even though I am not sure how they extend that to a residential garage. I have seen it applied tho. Usually receptacles are chair rail height in the ones I see, for whatever reason. I always assumed physical damage more than an article 500 issue.

Greg Fretwell