Can't speak about the 747 but it sounds to me as if these guys are lucky. With lower by-pass military engines optimised for high thrust and response it is quite common to find that idle thrust is too high and the brakes have to be used to maintain a reasonable taxi speed. A Hawk will stabilise at about 50 knots on a level runway at idle if you just let it go. Harrier pilots will often rotate the nozzles to 40 or 50 degrees to kill the effect of idle thrust while taxiing.