...Hmm. The gospel writers recorded Jesus as believing in Satan.
So what? No doubt many people of his time believed him to be an entity. Just because Jesus believed something to be true doesn't mean it was!
Actually, again you are making a positive claim, i.e. that Jesus was not the Son of God and did not know whether Satan existed. However, the reason I pointed it out is because BA's claim about Satan seems at variance with his belief that Jesus being divine unless, as appears to be the case, he thinks the gospel records about Jesus are inaccurate on this point. IIRC, he also thinks they are inaccurate in recording Jesus' views on judgement and, it seems, on Jesus' understanding of who God is.
Now it may be that I've misunderstood some of what BA believes, but I am sure he will correct me right if that is the case.