What kind of platform are you using? Your own design, the CYUSB3KIT-003 EZ-USB® FX3™ SuperSpeed Explorer Kit, or something else? (It all depends a little on how the UART lines are hooked up).
I am using the CYUSB3KIT-003 EZ-USB® FX3™ in fact
In that case the FX3's UART lines are hooked up to the debug chip on that development board.
And that debug chip will route everything your write to the UART to that chip's serial-to-USB port.
This is all nicely explained in chapter 3.7.1 of the Superspeed Explorer Kit User Guide.