If the gospels aren't historically accurate documents then everything which Jesus is recorded as saying needs to be seen more as which words each writer wanted him to say. Think you have to split the biblical Jesus from a historical Jesus. The biblical Jesus may or may not bear some or any relationship to the historical Jesus. The biblical Jesus is, very arguably, a construction of decades later - which is where mythicist ideas tend to mesh with the scholarly consensus. eg the development of ideas about the nature of divinity.
So not sure a biblical Jesus sounding eerily similar to a Jewish Rabbi writing in Babylon either proves or disproves a historical Jesus. Where it gets interesting is whether the person writing down the sermon on the mount knew of that Rabbi's thoughts. There's a similar argument around the writer of Luke/Acts and Josephus. Atwill gets it backwards. The argument runs that it's not Josephus writing Luke/Acts but that portions of Luke/Acts seem to indicate its writer had read Josephus - and was in fact copying parts of Josephus and making factual errors based on Josephus' works. It's all interesting for the origins of Christianity and of the writings of early Christianities. But not so much useful for saying whether there was a bloke who did walk around first century Palestine claiming to be a messiah (or not?) and ended up having followers who, after he had been executed/died, thought he was God.