Super-fast USB communication; how is it possible?
Is it possible to get these speeds? How?


Not without a 32-bit Arduino with a USB interface.


  If it's, why is it not in the default Arduino library?


Because you can't use it on an 8-bit Arduino without a USB interface.


  In case there's no easy answer for the other questions, is switching from UART to CDC easy, so that I could continue my testing?


Yes, get a 32-bit Arduino with a USB interface such as the Arduino Due that the poster of that thread used.