That's likely a false statement, even if unintentionally so.
If the t-shirts print is offensive enough then I'm sure a strong enough argument can be made. After all, how are a "bomb" bluetooth name or a "free palestine" wifi ssid much different from a t-shirt with similar contents?
Try wearing a t-shirt with "I'm carrying a bomb and will blow this aircraft up" and see how far it gets you on your next flight. The crew (including the PIC) won't be amused.