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.

Dr Hillman

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

Dr Hillman 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 Dr Hillman 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

The fool defines monism themself, or in terms of other people's definition of monism, rather than understanding how Ammon defines monism.... The fool, is missing the point. The forest is being ignored, fixating on a single tree.... Grok the bigger picture here, what Ammon is communicating, don't get hung up on semantics

We see others call out the definition of monism that Ammon uses, pointing to it while using a rigid global definition that disagrees with it.

But there's an art form in defining it

How is Ammon using it, what's the meaning he is trying to convey, even if he is using it unconventionally, what's he trying to tell us?
What type of monism is he referring to? In Ammon's case it's not some black and white literal definition, but one informed by philosophy, and it's exactly black and white thinking that Ammon is saying is monism. Regarding ethics and metaphysics

And ironically those who don't understand this nuance, are being very black and white and failing to see the grey.