I know what a relais de poste was.
In the Middle Age, many rural and not really important castles were small. Some of these castles even didn't have a turret.
These noble families were very often quite poor and lived like their peasants.
Château de Montmagner was one of them. The building was used as a Relais de poste much later. Certainly from the 17e century on.
Locally, it's known and called "château" de Montmagner.
NB : I studied medieval French history at the university...yeaaaaars ago
Forgot to say that Relais de Poste didn't really exist in 1340 something. It's a Modern Era thing (from 17e century on).
So no, it's not disingenuous to call this building a Château.
Many people seem to have the wrong conception of what a Château is.
For example, for many French, the late 19e century castles (the ones that the Brits buy like Basmaignee or La Motte Husson) are not really seen as "real castles" for us. More like a folly for rich Nouveau Riche families (merchants, industry, etc).