I'm not sure my "Wayback" machine can go back that far!
I would guess that the section is not there because of anything specifcally 'wrong' with rubber cords. Rather, I would expect that limitation to be more one of 'definition,' one that goes back to the very beginning of the NEC.
That is, I would guess that it was recognized that there was a legitimate role for 'temporary' wiring, and that it would be silly to ask 'temporary' wiring to follow the same rules or use the same materials as 'permanent' wiring. From there, the code went on to describe various methods- and 400.8 simply classes cords as a 'temporary' method.
Beyond that, the NEC is a nest of grammatical gremlins, whose breeding is nurtured by the lack of an overall foundation and the complete ignoring of the declared scope. When the rulemakers don't follow their own rules - or lack such to start with - it's small wonder that the 'codemobile' weaves about like a drunk driver on an oiled glass plate.