The runway was extended significantly immediately after the 1982 conflict in order to accommodate F4s. This was predominately done use MEXE/PSP metal 'matting'. Once Mount Pleasant was open, there was little/no need to keep the long runway.
36/18 is a tad under 300m short; plenty for an Islander and indeed the Do228 that used to float around. 27/09 is about 1200m.