I don't think that's true and Googling for it didn't come up with anything. Croatia and Montenegro do share a border but that must run afoul of the map creator's "no exclaves" rule as that bit of Croatia isn't contiguous with the rest of Croatia.

Yourube search for strange borders. The number of enclaves is staggering. Upon further inspection... fail.

The map explicitly says that it ignores exclaves.