As the 2nd article points out, they are capable of regenerating after deletion with files that go to many parts of the system.
Safari/Browsers – can’t delete cookies
Evercookie
The myth of the dangerous cookie
Quit Safari.
In Finder go to Go menu and then Go to Folder.
Copy and paste this ~/Library/Safari/Databases.
Delete the contents of the folder.
Launch Safari again and check if all is ok.
If that doesn’t work, copy and paste this ~/Library/Safari/LocalStorage.
Delete the contents of the folder.
Then go to Safari/History and delete any entries for the related websites.
Test.