Monism

Plato and Metaphysical Monism

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.

Plato's Form of the Good is a metaphysical monism — an ultimate source from which all intelligibility and value flow.
Everything derives meaning or being from a single, highest principle (the Good).
Abrahamic faiths (especially Christianity via Neoplatonism and Augustine) absorbed this notion: one God as the ground of all being.
So this is about ontology and metaphysics: the nature of reality and what everything ultimately is or comes from.

Ethical monism

Ethical monism — the belief that my ethical code is the right one for everyone — is basically moral monotheism

The analogy to monotheism is rhetorical: just as there's one God, there's one true morality.
This is a position in metaethics, not metaphysics — it deals with what is right and wrong and for whom, not what is or what all things come from.

Ammon

We also use this term because Ammon uses this term, to describe the monotheists.

Ammon seems to use Monism to means belief in the Abrahamic God as the one true/only god (metaphysical monism above), or any God that forces their God and deity as a state religion via force (ethical monism above).

It’s Abrahamic religions which Ammon means as monist, and also (of course) Aten-ism.

Or, feel free to Ask him... gather the info, and send it over to us!

Bottom Line

You can interpret monism as a stand in for Monotheism, which may be more familiar to you