thank you for confirming the current hours, Peatisback!
However, my guess is that the idea of using the lounge as an "arrivals" lounge simply doesn't see a whole lot of demand given the logistics (the customer could get a gate pass from LH if the airline is willing to issue it, but he/she would have to re-clear security... hardly worth it for a free drink or quick snack??)
It is on the way from the UA gates, and is a nice place to stop and enjoy the single stall women's bathroom, or a decent coffee, or just a quick glance at the newspaper from 'home' when arriving from ORD (which no longer has German papers in the B18 RCC by the LH gates)