But in a mystery RPG, there is no body in a parlour.I don’t think we need to get into higher level math or philosophical questions about reality to understand the idea that in life there can be a real body in the parlor and this an objective fact.
There is a statement uttered by the GM: The body is in the parlour. And the players' reasoning is from that statement, not from examination of a body.
This is (as I read it) the crux of @hawkeyefan's focus on the (mis)application of the adjective "real".
No one denies that players can reason from statements that the GM makes. Nor that the GM can be constrained, in what they say, by adherence to pre-authorship.
My point is that reasoning from, and with, statements about fictional states of affairs can be constrained in other ways too. Pre-authorship is just one mode of constraint. RPG design started to really work this out somewhere between 25 and 35 years ago.