How deep into conspiracy theory territory is positing an entire secret market of buyers and sellers of stolen artifacts? Because that seems much deeper than simply stealing jewels to make a political statement.
Well, we do know (1) that fences' [1] are a thing and have existed since time immemorial, (2) all kinds of stolen jewlery and art pieces found buyers all the time, and (3) the majority of heists and burglaries (as opposed to symbolic or terrorist acts) are simply motivated by the money. Very few thieves ever proceeded to made some political statement (e.g. the Bonnot gang).
If it's intended as a "political statement", then they chose a very roundabout and ineffective way to go about it, not to mention one that could land them in jail. They could just as well storm the palace or the place where the goods were kept and put out some banners or make some (non felonious) noise without stealing anything (much less covertly). The "happening" statement in such cases has been done lots of times (has happened in the Luvre, in Notre Dam, in various palaces, in Italian museums, in official conferences, etc. Pussy Riot are a good late example).
How do you view acts such as statue toppling/removal/theft[0][1][2]?
[0]https://en.wikipedia.org/wiki/Firdos_Square_statue_destructi...
[1]https://www.cbsnews.com/news/8-now-charged-in-toppling-of-du...
[2]https://www.nytimes.com/2017/08/17/world/controversial-statu...
It's as if people never heard of art and jewellery heists before...
Are we expected to believe that high-value buyers wouldn't authenticate the art they buy?