I fear you are right, but I hope not.
I think the scarlet gospels was written by Barker, but in a completely different form, and I think it was a chaotic mess. Miller took that material, excised 2/3 of it, and assembled a novel from it. So I wouldn’t go so far as to say that Miller wrote the book, but he certainly was responsible for assembling it from raw materials not fit for publication.
If we see another book from Barker, I think it will be done the same way – Barker does a first draft, and Miller or another author makes it publishable. Abarat 4 Has been almost done for about five years now, and it’s hard to know if we will ever see it. There’s probably something there, but I expect it still needs a lot of revision to be published. I suspect revising his work is Barker’s least favorite thing to do, Which is why he can tease people with photos of manuscripts in various stages of development, but nothing gets published.