UA sells plenty of tickets with airlines where they do not have a MileagePlus earnings relationship, including Hong Kong Airlines (HX). That said, I don't see where you'd be flying HX on that itinerary, so maybe it's somebody else?

Anyway, this is a situation where you can theoretically double-dip; you can get miles for the flight you flew, with whatever partners that airline might have (sadly, HX doesn't seem to have many), and then after the fact, assuming that the miles and PQP don't work out in your favor, contact the MileagePlus Service Center and request Original Routing Credit. On a UA ticket, for a UA-caused irregularity, that should be a straightforward request.