Why is that a requirement? What theological problem does not knowing when he fell present a young earth creationist? He fell before his appearance in the garden ... does it have to be more complicated than that?
Because the devil rebelling is why we have this 'flesh' age. And God had Moses pen the rebellion took place in Genesis 1:2 before the 'days' of earth clean up and the creation of flesh bodies wherein the souls that already existed were placed.
Christ Himself told Nicodemus, (John 3) that only those that were born from ABOVE not AGAIN could 'see' the kingdom of God. The devil will never be offered that opportunity as he was before this flesh age sentenced to death. And the devil is called 'death' in Hebrews 2:14. And in verse 10 of John 3 Jesus answered and said unto him, "Art thou a master of Israel, and knowest not these things?" So it was NOT a new thing that Christ was teaching.