I read the same, but /u/solitary_kidney on Reddit said that they have had many very nice thistle rennet cow's cheeses in Greece. I'm really not sure. However, given that they never add salt to their cheese (for reasons that one can guess), it may be that they are just used to bitter cheese. Historically the Romans used fig sap as well, although the descriptions clearly say that renneted cheeses taste better. I think NEC sells thistle rennet and I've been tempted to give it a try. Fig trees are all over the place here and if I can get a clear description of how to treat the sap (I'm worried it may be toxic) then I will also try that some time.