One note, the authors claims Arrian wrote The Discourses over four books, but in reality it was eight and only four survive. This calls into question the scholarship here.
As far as I was aware, Marcus Aurelius had a reputation as a wise and temperate philosopher king in his lifetime, and was later considered the last of the "five good emperors". If he was a warmonger then let us remind ourselves that even in the modern day our leaders have spent the last hundred years waging war somewhere or another in order to maintain a sphere of power too. And Meditations was in reality his private diary - like any regular person would routinely keep - and was not intended to ever be published. Characterizing it like she did is uncharitable at best and misleading at worst.
(Meditations is also an excellent read for those interested in Stoic philosophy.)
This is less of a compliment than it may sound like, because it is his own fault that he was the last.
The previous good emperors each selected their own equally good successors (by adopting them, so that they technically became their "heirs"). Marcus ignored this tradition and left the empire to his crazy biological son, who immediately ruined it.
You can look forward to some acerbic letters about this article in the next edition (and maybe even a response from Willis too).
So four books? Seems to be a bit of a distinction without a difference.