It was mainly a silly comment for the benefit of anyone who's ever seen the movie The Princess Bride.
"Inconceivable" could replace "WTF" sometimes, but only if "WTF" is intended to signify that the user simply can't believe what he just read/saw. If "WTF" signifies "This is just stupid", another use I've seen for that acronym, then replacing it with "inconceivable" would be incorrect.