Personally, I have found the fact this researcher himself is not complaining about this and remains anonymous to be pretty suspicious in itself.
Instead we have a French beurocrat complaining about it on his behalf himself pushing the bad messages found narrative. This all smells of cover-up.
A plausible explanation would be that the US knows confidential information ended up in France and the person who was denied entry was the only plausible vector but was not caught red handed. Instead he was shadow banned and was nabbed for interrogation at the border where he confessed. And it could well be that the border agents scraped together a story about his messages as an excuse to bounce a persona non grata to keep the diplomatic issue quiet because banning a guy for Trump hate is a better diplomatic choice. (i.e. what is to be gained from holding him vs letting France burn him for getting caught). This all seems extremely plausible to me.
In any case there's obviously more to the story and that's the point. Not knowing who this guy is really underscores there's something diplomatically delicate at play here and the US has sent France whatever message it needs already IMHO.
Put another way: if you are affiliated with France's nuclear weapons program maybe there's something work-related going on between France and the US. That's how I interpret this story.