That's an example where the Routeing Guide in Detail contradicts the Routeing Guide instructions, so it depends on your interpretation.
Where does it contradict this? The Routeing Guide in Detail is inconsistent with the instructions in various places, but it only refers to doubling back:
On page 4, to prohibit using an associated routeing point which makes the journey double back through the origin or destination where the fare does not permit this (whatever this means, it's not an issue here).
On page 8 steps C and D, to prohibit tracing a route on the maps which doubles back.
On page 10, to say there may be easements which allow doubling back.
On pages 15 and 17, to prohibit tracing a route on the maps which doubles back (again).
None of this suggests a reason why York-South Milford might not be valid via Leeds. The step C bit is ambiguous, but it's hard to read much into it regarding stations which aren't routeing points when it's specifically discussing a journey from Sheffield to Inverness.
In any case, Johngill100 doesn't need to use this ticket at the barriers in Leeds. The guard on the Leeds-York service doesn't know where Johngill100 went before boarding the train, and probably doesn't care given that there's no way to check. As such, I wouldn't expect this to come up at all as a problem.
I was concerned that a guard may feel that a customer is only allowed to travel York-Micklefield-South Milford, as would have been the case using recent versions of the routeing guide published before 26/9. That's why I suggested an itinerary.
I don't know how staff have been briefed about this sort of thing, though.
