Still having trouble with the difference between "this power modifies an existing ability to do X" vs. "this power provides an additional ability to do X"? If you examine your alleged "gotcha", you'll find that this is the difference between them.
What difference?
Threatening Reach in the MM does not state that the way OAs are provoked is changed. Its benefit is the ability to make OAs within your reach. But unless the way ranged attacks provoke OAs is changed, someone only provokes an OA with a ranged attack if they're adjacent when they make it.
Now, if "You can make OAs under condition X" includes the understanding that "... and condition X provokes OAs", then fine - a ranged attack made within your Threatening Reach provokes an OA from you, and you can make that OA.
But by the identical logic, Warpriest's Challenge - "You can make an OA against an adjacent opponent who shifts" includes the understanding that "an adjacent opponent who shifts provokes an OA from you".
You can't say that "You can make an OA when..." changes what provokes an OA on one page, but "You can make an OA when..." doesn't change what provokes an OA on another page.
Threatening Reach and Warpriest's Challenge both say "You can make an OA", and neither of them explicitly extend the scope of what provokes an OA. So either both of them implicitly extend that scope, or else neither of them do. If the MM text for Threatening Reach doesn't implicitly extend the scope of what provokes, then the PHB text is required in order for the ability to be of any use. If it
does, then so does Warpriest's Challenge, and the shift is what provokes the OA.
-Hyp.