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 ()
(message "%s" (this-command-keys)))
(global-set-key (kbd "C-<f3>") #'bababa)