I think the (OP) article has screwed up here. The article, and I think its original source, name a particular set from the theater as the palace's Mona Lisa. But the article has a picture of the theater itself, and even misnames the theater after the set.

Tatler source: "This includes machinery that causes a tree to rise from a trapdoor and three sets – a simple interior, a forest and a temple of Minerva – the latter being the oldest intact decor in the world, dating back to 1754 – ‘our own Mona Lisa,’ said Masson."

OP article: "What is it? The Temple of Minerva theater set (c.1754) from Marie-Antoinette’s private theater."

OP caption on picture of theater: "Temple of Minerva theater (c. 1754)"