I concur. Technically speaking, the backbeat doesn't have to have any swing at all. It can be anywhere in the spectrum between straight and swung, so long as the accents are on 2 and 4. But if you're talking about Bonham, as people do, and why his backbeat felt so good, it's because he was slightly swinging it.
The backbeat evolved out of jazz drummers playing 2 and 4 with the foot on the hi hats along with the jazz ride cymbal pattern. Then, as boogie-woogie piano players developed their style, the eighth notes started straightening out and, with the influence of things like 2:3 clave as in New Orleans second line, the backbeat began being accented on the snare drum. The lilt of the blues comes out of the swing-shuffle, drawing its accents on 2 and 4 from the New Orlean influence and its orchestration on the kit from jazz. It was all the result of a meshing of African polyrhythms and European instrumentation/harmony/melody in the US of the late 19th and early 20th century.
So, the backbeat doesn't have to swing, ever, but that is where it came from.