This came up a couple years ago on the TDPRI and a guy posted what seemed like a credible story, the jist of which was that David Nelson didn't get a stringbender until several years after the tune was recorded...which makes sense because at the time (1970) there were not many people selling them...no Fender, no hipshot, no Parsons-White, no Glaser, etc. Dave Evans-PullString was starting to make some but apparently didn't make one for Nelson...Evans benders are prominent in the early Eagles-Bernie Leadon had one, and some players built their own-Bob Warford-backed Linda Ronstadt was one ....
So my guess is he played the solo "digitally"-with his fingers...its one of those solos that is easier to play with a string bender...and maybe a bender was used in subsequent performances...but not on the record....
Ive been playing a stringbender for about 20 years...my Nashville West "Clarence " clone from a festival gig a few years back...
Dire wolf is the queen mother of changes.....each verse has a different progression