I never even noticed that it's not native until I read your comment, and looking carefully at it now the only difference I can notice is the corners are square (and that is certainly nothing worth complaining about.)
Maybe I'm just an oblivious idiot, but it sure seems to me that a lot of MacOS users on HN love to blow trivial matters completely out of proportion. Application lags for a split second? "Utter garbage, the developers should be shot, I'm going back to Safari." Where is the perspective? If there is something glaringly broken about these context menus, by all means correct me. But to my eye they look damn close to native and they seem to work perfectly fine.