[...] I find this whole thing somewhat unconvincing [...]
That rule was settled 20 years ago by the French administration:
- there was maybe something like that scheduled for all/severall European members, but it was "buried" in the meantime?
- the reasons given by the administrations for their acts are like politician speaks: hardly (if ever) convincing
[...] and really a French administrative ruling would only apply to French models anyway.
If I'm not mistaken, due to taxes and accounting regulations for French settled companies, it applies to all vehicles from all makes sold in France, as long as the seller's (seller's, not maker's...) head-office is in France.
Practically, if changes occur in July 2020 for a vehicle, that makes the concerned vehicle a "July 2020" one, for the French market, that's all. This is the same in most countries: all companies have to cope with the local rules, and the
January 2020 Renault Trucks T is probably seen as a 2020½ model in Germany...