There's been a large "friends of the comic" group since day one, there was (and likely still is) an active IRC group that included Munroe that discussed old comics and generated ideas for new ones, the alternative text behind the comics and related facts sheets get a good going over by a small cadre of friends and regulars with an engineering | physics | maths | chenistry, etc bent.
(And besides - if Randall f*ing Munroe contacted a few people, who were experts at $Foo, $Bar or $Baz, with "I've got this really cool idea, but I'd need some help..." - how hard would it be for him to get some "yes" answers?)
