Monism
Plato
Monism is like the Platonist crud that posits everything comes from some "oneness" of good, or "one good" - The Abrahamic faiths are just knock offs of Platonism in this respect
Plato's concept of the Form of the Good can be seen as a monistic element, providing a fundamental unity to the plurality of Forms.
Ammon
We also use this term because Ammon uses this term. Let's put some trust in this PhD in classical philology with +35yrs experience, he's deep in the subject, and is likely speaking from some nuanced perspective, or timeperiod, that you're unfamiliar/unaware with.
Generally if Ammon's using a term often, and you wonder if it's wrong, there's typically some background to why. Seek to find out what's behind his comment.
Ask him, or ask around and someone will explain to you...