Perhaps I misunderstood the original question. All I meant was that GPS or not, RNAV or not, the pilot is required to fly the published procedure, meeting all of the altitude restrictions and other requirements depicted. And of course, in the visual segment the crew is required to comply with 91.175(c) for descent and operation below DH/MDA, as applicable.
For now, we know from the NTSB, citing CVR evidence, that the crew, Captain flying, were using the LOC 18. Until further NTSB revelations from the available data, we are left to wonder why they were so low as they were, below the nominal 3.2 degree vertical path indicated by the PAPI and why they apparently had such a high vertical velocity to generate a "Sink Rate" alert 7 seconds prior to CVR sound of impact.