The ingredients for a conspiracy theory are all here for anyone who wants to discredit Roddenberry. There's no proof and no real accusation from Solow, just a hint that suggests someone else might have had input. It's hard for me to believe that if Roddenberry had screwed Peeples on The Cage script that they would have worked together on anything else afterward. If you apply Occam's Razor the simplest answer is that Roddenberry wrote it. Could have gotten advice or help from a friend? Sure, but not so much to qualify for co-writing credit and there's no reason that I can see to think Peeples felt that way. I'm not a Roddenberry apologist by any means. The more I learn about him the less I respect him, but this accusation seems like a reach.
Well, let's just point out this very simple fact. Solow was there. We were not. He had a very involved position on the creation and production of Star Trek. Its possible he still has an axe to grind, but come on... Why bring it up 20 years after Roddenberry's passing if there's not some basis to believe that maybe, just maybe there were other parties involved? I don't believe from what I've read of Solow that he'd ever be quite that vindictive.