The docstring says:

Unlike ‘this-command-keys’, this function’s value
does not include prefix arguments.

However, the function ‘this-command-keys’ also does not seem to return
prefix arguments. I tested this by evaluating the following code and then
typing C-u C-F3. (the output is just [C-f3])


(defun bababa ()
  (interactive)
  (message "%s" (this-command-keys)))

(global-set-key (kbd "C-<f3>") #'bababa)