Thanks for sharing this essay or rather long commentary. It’s interesting, but of course I would have liked to see specific sutta references, but that was probably not the purpose of the text.
I can’t speak for the Agamas of course, but some claims about the Pali canon are doubtful. I think I can refer to my nimitta source collection as a reference where I collected all occurrences of animitta samadhi as well (3b).
As the sources show animitta samadhi (whatever it is) can be located rather clearly:
a. it’s beyond the fourth jhana
b. when the arupas are mentioned it’s beyond the highest arupa
c. when the cessation of perception and feelings are mentioned it is below it
d. it appears a few times at the same level together with suññata (emptiness) and appaṇihita (desirelessness)
Nowhere it is mentioned in EBT that it could be unskillful, quite the contrary, it is praised, or recommended, or described as a state where the Buddha chills out.
So maybe in the Agamas it has a different character, but at least in the Pali suttas these are pretty much its features…