I think it's saying that McDonalds has lost the "Big Mac" trademark in relation to chicken sandwiches. Presumably they still retain it for beef sandwiches.
At least in the UK, they've sold a "Chicken Big Mac" as a special menu item twice now
Right? Imagine someone asks you what you'd like to eat. You say "I'll have a Big Mac." Would one assume you meant a chicken sandwich? Or a salad? No, people would think you're talking a burger, a specific kind of burger.