Most of the time it was implied in TOS that the warp drive was the main means of propulsion for any travel speed, with impulse as a backup - which fits in with Timo's observation from Obsession (where Scotty seemed to think the impulse drive wouldn't be needed at all). I'd say that some writers confused it with a secondary or emergency power source. Although, I suppose it could be argued that the impulse reactors could be used to power other ship functions such these situations, freeing up more warp engine power for propulsion.