Yes, F fares to Hawaii buys a seat, not a particular type. UA does not guarantee the lie-flats to Hawaii in writing. Some routes are guaranteed lie-flats by virtue of a/c required to operate (eg EWR-HNL), but west coast flights allow 737/757 swaps. Sometimes one can convince an agent to switch to a flight with lie-flats, but if they reduce lie-flats from West Coast to Hawaii, there will be that much more demand for them on the remaining flights. Plus the schedule might not work for some.

I do think UA (from my observational experience) realized a bit more pre-pandemic for lie-flat flights scheduled to operate between west coast and Hawaii, and a/c downgauges then were pretty rare. Paying a little more for those flights were a little bit of a crap shoot, though - the lie-flats weren't guaranteed.