RE: flet is obsolete, but...

[Drew Adams]

>   (defun my/revert-buffer ()
>     "Unconditionally revert current buffer."
>     (interactive)
>     (let ((revert-without-query t)) (revert-buffer)))
> 
> but I don't understand why it does not work:

Others have explained why it does not work.
Here's another way:

(defun revert-buffer-no-confirm ()
  "Revert buffer without confirmation."
  (interactive) (revert-buffer t t))

(I bind it to `f5', in keeping with the MS Windows use of that key.)