Why not in the case of aircraft?? I presume this means the aircraft's own engine, but what about another aircraft taxiing past you??
I'd say this means other non-aircraft engines near you. You need 6 metres from stationary aircraft while refuelling, and this stationary aircraft can start its engine(s) without breaching 4.4.3 if it's 6+ metres away.
As for minimum taxi distance, it's not really specified. Those regs only mention "stationary aircraft" and engines not beloning to aircraft. Although the "stationary" bit might simply be due to the fact the person conducting the fuelling can't be responsible for aircraft movements near their aircraft - but they can be responsible for separation from parked aircraft.
Taxiing within 6 metres might technically be legal - although if you bring the aircraft to a stop, it's immediately illegal
