I am more inclined to believe Nigel played a strong but still developing engine.
If you listen to Fischer in his Iceland interview it is clear he thought chess was played out.
He played the Greenblatt Program in 1977 and then Spassky in 1992.
As far as I am concerned Fischer fell in love with chess aged 9 and due to an exacting streak dropped out of the game. Spassky was kind enough to play Fischer giving him some good money but sadly the US Government saw this as sanctions busting. Away from the chess board Fischer was a disaster.
An astonishing Blitz game of Fischer vs Short
Today clearly not, back when Short played the games, who knows. The guy whose name I forget found a way to greatly optimise the tree search for the move.
Fischer did not like computers, did not own one while in Iceland, which blows a big hole in him playing online.
I think its Fischer, Short was clever in asking if he (Fischer) knew this Mexican player and Fischer he did, only Fischer would of known this unknown player that he played. The game had many of Fischer's characteristic, good sound positional judgment and great endgame skills, I am not referring to the opening but how Fischer was able improve from a bad position, it take sound judgment to improve a position and back 2000 the engine had horrible endgame technique. Short would of been angry if someone was using a engine, he knew it was a human player he was playing against and rightly concluded it was Fischer.
https://www.youtube.com/watch?v=7B9p2PrsKWY
After just 5 moves, who would have given White any chances vs GM Short?!?