I'm going to go with no, since that step is not transferring data to a human, it's transferring it internally within the computer.
UI can refer to either the medium, such as a visual display, speaker system, or keyboard, and it can also refer to a specific layout of information (like the Qwerty layout, or a webpage layout). I wouldn't consider the USB protocol UI just because it can transmit HID Events, only the keyboard or mouse as a whole is UI.
You could almost call HID events UI, but I'd still argue they're more of a computer-device interface than a human-device interface