This may have been Bannons plan all along, I never fully trusted him and I sort of followed him from the sidelines, he always seemed shifty to me. I don't trust shifty people. Even when there were times he seemed to be correct on policy, I was never fully convinced of his loyalty and this seems to be all by design to get Trump supporter behind him.
After he was canned it weakened his hand. The fact that he backed Moore and lost Alabama proves that noone really listens to him, and it ruined his entire plan to do this little outburst for the release of this book. Bannon lost all momentum after Alabama which is why he hid under a rock for awhile, now he pops up and takes some shots to rile up Trump and deflect attention from the problems going on with the Russian Witch hunt.
I say this, Bannon will get his due, because Trump doesn't let such attacks slide. So I hope for Bannons sake he is squeaky clean and has tied up all his loose ends. You can be sure people will be sniffing around and will find something. Even before the presidency there were very few Americans with the public profile that Trump had, and now he is president so he's in a pretty strong place I'd say.
Maybe I am totally wrong and this is all really deep samurai dishonor type revenge on his part, I would actually respect him more if this was the case even as I don't like his efforts, but somehow I doubt this, there is something more here. Just the way he entered his team so late (August 2016) and even when obtaining such an important role in the WH he did all he could to disrupt with no interest in any outreach, suggests something is amiss.