can you elaborate on that?

A triple in the graph is a fact regardless of anything else. You need 4D ontology or n-ary relates or event based modeling or ... Can't annotate existing triples to do the same thing. (Sim. its an antipattern in LPG)

How can it be anything more than a proclamation by its author/publisher/etc.?

Do those things all have a responsibility to maintain the context in which each assertion was originally found, i.e. who asserted it?

I agree that a random RDF file found on the web should not in general be considered as a claim by whoevever "owns" that file (for some definition of "owns"). .../...

However, the question of monotonicity is orthogonal to that. Any RDF file makes a set of statement, and entails every subset of these statements. .../...