one could argue that many castles like Versailles are museums too
I note that Fontainbleau's web site address calls it a "Museum" - http://www.musee-chateau-fontainebleau.fr/spip.php?lang=en
The Plantin Moretus claim as written is clearly specious - instead of claiming "Museum Plantin-Moretus is the only museum in the world on the UNESCO World Heritage list.
" it should say something more like "Museum Plantin-Moretus is the only building in the world on the UNESCO World Heritage list which was built for a purpose other than as a museum but is now called a museum in its title and is on the list as a single building..........."
But that doesn't sound anywhere near as "impressive"!!
The UNESCO web site contains this piece of informtion about Museums on WH Sites -"Around 8,000 museums are said to be located in either cities or sites classified as World Heritage"https://whc.unesco.org/en/world-heritage-and-museums/
The same figure with a bit more background is cited in the book "Museums and World Heritage sites - A New Path to Shared History
" (Wiley, 2015). This states -
"One of the most effective mediators of communication about and presentation of World Heritage Sites should be the heritage institutions located in and around these protected zones. Although no official source is available there are said to be about 8000 of these institutions including storage facilities, archaeological conservation centres, interpretation centres and visitor centres
As if being on the WH List isn't enough in itself, some sites just love to claim even further "uniqueness". Bath used to claim to be "The only complete city inscribed on the World Heritage List
" on the basis that its "boundaries coincided exactly with the boundaries of the (former!!) municipal city
I wrote to them and pointed out a few other contenders and they "reduced" the claim to "Bath is one of only a handful of city-wide World Heritage Sites.