Earlier quoted context omitted.
Yes, the expressions “moving” – “not-moving” are rather misleading. The actual distinction according to this theory (or to my favourite flavour of this theory) was: “able to act willfully” – “unable to act willfully”. The distinction was quite logical. Cp.: (A) “Susan runs down the hill.” (B) “Water runs down the hill.” (C) “Susan drinks water.” A speaker of Proto-Indo-European might have said, learning English: “Thi…
> all over the old IE languages, neuter nouns have the same form for the nominative and the accusative The lack of distinction between neuter nominative and neuter accusative is not just true of nouns in old IE languages. It's true of all grammatical categories in virtually all modern IE languages too (with some very obscure exceptions). Consider German articles der/den vs. das/das. Or English pronouns she/her vs. it…
It is wonderful that this formerly semantic system (if the theory is correct) has left its formal trace over millennia and a range of thousands of miles.
Such preservation was likely in so far as otherwise a new suffix-inflectional distinction had to be introduced in order to distinguish the nominative neuter from the accusative neuter; and in the history of the IE languages, suffix-inflectional distinctions are rather removed than built.