See  for the thread that engendered the mythos (recirculated in ju'i lobypli 16 and 17.) 'Jimbobs' became an term used to designate the community of active Lojbanists of around 1992, including Nick Nicholas, Ivan Derzhanski, Mark Shoulson, Colin Fine, etc.
Some explications of the email:
- El Zamenhofo refers to adherents of the traditional Esperanto ideology, so named after the language inventor, Zamenhof, with a sideswiping allusion to Harlow's reference to El Capitan.
- Il Silferissimo is an allusion to Giorgio Silfer, an Italian Esperantist intellectual, and one of the primary advocates of the doctrine of Raŭmismo (Esperanto-language discussion), asserting that Esperantists should emphasise their achievements as an autonomous language community, rather than evaluate everything they achieve relative to the 'final victory' (finvenkismo, pracelanoj) of the universal adoption of Esperanto.
- La Danĝera Lingvo details the political persecutions Esperantists have suffered over the years, primarily under the Nazi and Stalinist regimes.
- Schleyer was the inventor of Volapük, and the archetype of the dictatorial artifical language inventor.
- Bruce Jilson is Bruce Gilson, notorious at the time on the CONLANG list as an anti-Esperantist and advocate of Ido and other, more naturalistic conlangs. He and others worked on a naturalistic loglang eventually called Voksigid.
- Carter Jimbob is Jim Carter, who has worked on his own Loglan-influenced artificial languages, most recently gua\spi; in gua\spi text he uses the name kartr jym, where y = English i in bit, not Lojban y.
- Ivan derJimbob, Mark Joulsonbob, Colin Jinebob, Nickbob, Bob !LeJimbob should be self-explanatory.
- John !McJohnbob is explained in the exchange.
- Jimbrownbob is James Cooke Brown. Calling him the First Digger is an allusion to the First Speaker of Isaac Asimov's Second Foundation.
Nick Nicholas originated the comparison between Lojbanists and stereotypical American 'rednecks' (Jim-Bob being an archetypal name of such), but does not remember why. Presumably he intended it as a mild protest at the Americanicity of Lojban Central, although most 'Jimbobs' were not in fact Americans.
I don't know which image is more interesting: Lojban intellectuals that advocate doctrines, persecution by totalitarian governments, or rednecks that drawl Lojban.
Er, Esperanto intellectuals, but I see your point. (Kinda...)