It's one of the antiquated requirements from the time when you could actually get your Austin 7 in a garage and people stored drums of parafin for their lamps and front room heaters.
Who stores flammable liquid in the house garage in quantities that would fill a typical garage floor to a depth of 105mm to breach a step - that's about 1.5 - 1.7m3 of liquid - 1500-1700 litres! I think there is 65 litres in my car tank. And how does a garage floor actually fill up to 100mm depth without the liquid soaking into the floor, evaporating away, draining between the gap under the garage doors, or just blowing the house up?
There is even a nonsense BRE study on the performance of a 100mm step. Not a 50mm step or a 150mm step just a 100mm step. The study has more holes in it than a typical garage floor. But has some nice pictures.
Anyway, just hope that your inspector is a bit clued up and pragmatic. 20mm is nothing in context of the risk a step is supposed to guard against - how will the floor fill up with liquid, where is the door, where is the garage door that this hypothetical liquid will drain under.
Jeez I was hoping for a calm Friday.