the question is not about monism.
if we are going to discuss monism, we will have to spend hours defining what kind. of essences, substances, etc. so it is a concept that traps us in things again. if there is monism of essences, is there one of substances, of realities? we can debate this forever. but we will never have an answer, because the definitions themselves are unclear.
so we must abandon monism for exactly that reason. it is an empty concept, mostly a Western concept, a way to try to divide oneness again, this time into categories, not relations. but now we are separated again.
let us cut to the heart of the matter. are there things, or no things?
if there are things, then there are objects and subjects. if there are no things, then subject is object. this is a way of seeing beyond monism.
we are in a position perhaps for the first time in human history to understand that question. we will probably need not to rely on old concepts, but to create better ones.
as for buddhism not being monist, it is certainly a rejection of dualism. whether or not that implies monism you can debate with monks from different schools forever. but that is not theology, not thought which clarifies.