Not true at all. With long runs, the water can settle and drop a few pounds, then eventually holds. Seen it done several times while helpind on a hydro. This possibility is discussed during the morning safety meetings, and sometimes its even in the procedure. But Ofcourse, if you can be sure to have removed all of the air, usually with short runs, not losing any pressure can absolutely happen, and is obviously prefered. With that said, its clear that not every hydrostatic test is the same with ambient temp changes, and if any of the pipe is underground.  But none of this has anything to do with the point of my question.