Dictating isn't collaborating, though. Those are completely different things, because the transcriber doesn't contribute anything creatively, just converts the writer's words to text. Dictation is simply an alternative to typing or longhand. Lots of solo writers have used it, including Cartland, Rod Serling, Erle Stanley Gardner, and J. Michael Straczynski.
So the method Shatner used to record his writing is beside the point. Either way, he wrote portions of the work and his collaborators wrote other portions, and they rewrote each other's words. That's how most collaborations work.
Thanks, Christopher, for your insightful clarifications.
I think part of what is going on here is some feel Shatner was a Johnny-come-lately to writing. You're knowledge comes across as pretty widespread so correct me if I am wrong, but Shatner was interested in writing before his involvement in Star Trek.
I seem to recall him mentioning in an interview somewhere that during his days of struggle he almost shifted focus to writing and directing to make endsmeat. Anyone else recall this? Wish I could recall a source, but there you go (for what it's worth).