I didn't watch the full video, but from what I did watch, I don't see anything that indicates the CDC serial capability of the ATmega16U2 chip is retained when using the video author's custom firmware. Notice that they specifically instructed you to upload the sketch before flashing the custom firmware.
You might consider switching to this ATmega16U2 firmware, which adds the same keyboard emulation capabilities to the Uno, while preserve the ability to continue to upload sketches to the Uno via the USB cable: