Duncan was likely talking in the longer term here as opposed to this being actively in the works at the time of the revolution. There were some ambition when the War of the First Coalition came around that the British hoped that with everyone attacking the French they might be able to keep some permanent holdings around somewhere like Dunkirk to have a toe hold on the continent. Likewise the Prussians had their eye on Alsace which was French controlled territory within the Holy Roman Empire legally speaking and they were after carving off a little toe hold on the west bank of the Rhine and not for the last time either. But there was not yet anything being so actively organised as the partition of Poland.

But while French power was evidently in decline before the revolution I don't think it was quite as far along as Poland-Lithuania but it wasn't as powerful as it once had been beyond the boarders of the main metropolitan France. In the 7 years war it had lost much of its territories in the Americans and India and even in Europe seemed powerless to act when other great powers were expanding their influence close to the French border such as the Prussian intervention in the Patriottentijd with the Prussians pushing out French allied forces with the French being willing to pay the cost of raising an army suitable for marching to go fight them over it. So while perhaps its a strong assertion that had the French Revolution not happened it would definitely have occurred shortly there after I don't find it entirely unreasonable to suggest carving new chunks off France might have been something European leaders might have begun talking about say ten twenty years down the line. Whether it would have ever come to be as fully consumed as Poland was is hard to say