Mmmhhh...... I'm quite sceptical.
It's a bit difficult to know exactly with he means as he's using vague terms for things which are very specific, but what he's talking about sounds like it could either be the Garde Constitutionnelle du Roi (personal guard of the king during the constitutional monarchy era, and obviously stopped with it), or what was successively called "Garde de l'Assemblée Nationale", then "Gendarmes Nationaux", then "Grenadier Gendarmes près la Convention" aka "Garde de la Convention", as it was at the center of the intense political turmoil of the period. The very political turmoil makes it a period quite difficult to sort things through, especially things that anecdotic. But anyway, I did my research, and I don't happen to fall on Mr. Comfort conclusion AT ALL.
First, what seems to be a fine exemple of the Garde Constitutionnelle sword... marked on both sides from the fourbisseur Coullier!!! Its authenticity leaves no doubt:
Then, an officer sword of that ever-changing parliament guard with a thousand names, and small size scans of an article in the Gazette des Armes depicting what is clearly the same type/model:
Of course, none of this has nothing to do with gardes tournantes, and the very explanation Mr. Comfort gives in regard to its origins contradicts everything I read from the French collector community (also that emphasis on naval use, which makes no sense: it was just so commonplace in both the infantry and light cavarly that it could have end up anywhere, much like a briquet).
Pétard and Ariès, in a leaflet dedicated to fantaisie sabers from the second half of the 18th century (in cahier XXII from 1974), show various gardes tournantes, but the early ones are more like split branches, like those we can see on Walloons and such. But they also give the following drawings in attachment. We can clearly see the beaded branch style (garde perlée) of germanic/eastern influence, and the well formed garde tournante mechanism. It's difficult to pinpoint exactly the decade during which this style was fashionable in France, but it surely was by the 1780's, and not so much by the 1790's. The other saber is more 1790's-ish.
But there also is a variety of other petits Montomrency à garde tournante, or other infantry or cavalry sabers with a garde tournante (and I mean that specific kind garde tournante, with the leaf lock and all) that can be dated to the late years of the monarchy, i.e. late 1780's, because they're otherwise similar in every other aspect to similar sabers of the period.
I don't think we will ever figure out who invented the garde tournante, if there ever was such a person, but we can safely assume that it evolved from previous designs (like those on the Walloons), and during a brief and intense period of time, a bit before the Revolution and during it (and you can read in periods accounts that people were really smelling something huge was in the air even before anything "serious" had really started), with an amazing technological coincidence as hilt fashion turned the way it had (many parts fashioned from sheet metal), making it extremely compatible with that specific garde tournante system, it rapidly gained a very significant popularity, and also fell in about a decade as the political situation changed as well as fashion in hilt designs. Not that it's only French, of course, but it's typically French, and I really think this can't be taken out of the equation.