Still amazed someone could fly for 4 hrs on an early era IRS with no GPS or DME updating, make up an uncertified approach and fly down to well below MSA. Drift on those old IRS must have been in the order of 3-6 nm an hour. So doing the homemade IRS letdown may have put him 12-24 miles off apparent track anyway with the IRS still within tolerances. And without DME updating who knew where they actually were.
Anotherday; I have no experience with INS/IRS systems on the DC10 but I do recall hearing they were very accurate, with crews expecting to be very close to track after a trans Pacific flight, certainly much much more accurate than you have supposed.
It's been a while since I read anything about this accident but IIRC the crew had identified their track visually, all be it incorrectly by mistakenly identifying similar looking landmarks. They were in their mind where they expected to be.
A letdown as you describe it infers descent to get below cloud. My recollection of the transcripts is the crew thought they were in VMC.