I think it's correct. They were lost to the general public and were considered missing. When a smaller yellow diamond surfaced there was speculation that it's the Florentine diamond that was cut.
I'm not in the jewellery business but I would be very surprised if the diamond industry didn't have a pretty good idea where things lay. That kind of multi-caret stone has a premium. You need to know provenance. The risk it's a blood diamond or stolen is very high.
Lost to wider society? Yea, I can buy that but it's a stretch.
It's not "lost" the way a medieval triptych hanging in an alms house for centuries and found to be worth millions is a "lost masterpiece" or works coming out of safe deposit boxes where some GI has hoarded loot after ww2. The titular holders decided to put a time lock on their deposit box but had clear title to the assets.
