Not.. really.

Whether or not to include an APEX fare (“Advance purchase excursion”), which is what you’re describing, has always been a market-by-market decision. It has never been the case that you would always get a discount for booking a round-trip, just that you often would.

To answer the question in your thread title: “In general, does United give better pricing if RT, rather than 2 one-ways,” the answer is that there is no answer. Sometimes two one-ways will be the same price; sometimes (rarely) they will be cheaper, and sometimes they will be more expensive. The only way to be sure is to price out each approach.