VL - yes the S61 ELS is responding to low oil pressure and the beauty of the system is that failure of one pump isn't enough to trigger ELS so lower than normal oil pressure is indicative of a pump failure. The design of the gearbox means that one pump is more than capable of providing lubrication and, if the other pump fails as well, the ELS takes on the job to get you home.
How is it that a 21st century helicopter, with the latest certification, can't manage the same? The weak link of any driven component is its driveshaft, vespel splined or otherwise, so how did the the engineers ignore that failure mode and persuade the FAA to do likewise? - there is little practical difference between not having any oil to pump round the MRGB and not being able to pump it because the pumps are inadequate.