I think it's pretty clear that Moorcock is showing how ambiguity is an important or even necessary part of the human experience. He himself doesn't know for certain whether he was kicked off due to

A) Kubrick not knowing for certain who he was, or

B) Kubrick knowing exactly who he was but aiming to prevent Kubrick's earlier proposals to collaborate with Moorcock from coming into public view.

I notice that two different commenters here have reached two totally different yet sensible conclusions, based on exactly the same evidence. To me, Moorcock seems to be staking his flag into the ground over the inalterable need for truth to be filtered and interpreted by the human psyche - and that if even direct and worldly concerns like this have no straightforward single answer, the idea that our species might encounter spiritual or transcendental truths without ambiguity over what they actually mean is pretty tenuous.

But I could be mistaken.

The original article was to have a voice-over that explained such things.

It wasn't spelled out, but I think Kubrick didn't know who he was, and just didn't want anyone extra on the set. Given how behind and over-budget the movie was, and how many people wanted to visit such an unusual (for the time) film production, I can understand that.

Film people hate having non-crew people on set, they get in the way, are a safety risk, and are generally distracting. It's typically intensive work with 12-18 hour workdays, and for all the creativity and artistry it's also a busy industrial environment. People who don't belong or get on with the film crew are generally not made to feel welcome.

Kubrick didn’t want it to come out, on the set, that he had asked other authors to collaborate with him.