From TheBigWhale:

The ANJ, France's gambling regulator, is expected to ban access to the site in the near future. The cryptocurrency prediction platform was a big hit during the US elections.

For context, the top of the Polymarket leaderboards are dominated by the Frenchman Fredi/Theo (he has more alts beyond this):

Is this report legit, or nonsense?

For reference, access to Polymarket in the US would clear this bar, even if plenty of Americans still use the site.