That is the thing that surprised me, and it didn't seem to make sense - why a single season (which would have been less than a 12-month calendar period) would have invalidated his HGP status. Wikipedia shows him spending a single season (in 2004) with the gloriously-named Wilmington Hammerheads...
Home grown status applies to players that develop through English and Welsh clubs only, so Murphy (who came up with Motherwell) doesn't meet the requirement.