I think they’ve guessed at the figures on that link.
That looks like a 10:00×16 tyre, which would be 36″, or about 1000mm, in diameter.
Reverse gear on a Land Rover is about 50:1, on an AEC Matador it’s about 80:1, so the Austin would probably be somewhere between the two, say, 60:1.
So that’s double the tyre diameter and double the gearing reduction.
The two cancel each other out, so overall, I’d say they’ve got the final result right.
I’ve had a flat battery on a Land Rover parked just off a track on a slope.
I used the starting handle to wind it the short distance up to the track so I could bump start it down the hill.
Only a short distance and not much of a slope, but it worked.