It wouldn't be that difficult to verify. Just cut it apart. It's likely just a grossly shaped chicken tender although I'm unsure how it being "white meat" matters...rat meat is also considerably light when cooked. Just cut it apart and see what's inside.
On another note, though, stories like this don't make fast food immediately negligent. I once opened up a peanut and found a worm (the species I'm not entirely sure, it looked like an earworm). Biology is fascinating, creepy and magnificent all at the same time. If a rat's alive, most likely it's somehow going to get into something, somewhere, although usually it won't. Just like you can't guarantee that a worm won't be inside a peanut, although 99.99% of the time it won't be in there.