I think that's just because she's insane, not because she couldn't get it. She might never have heard the thumping in the coffin (In the true ending, everyone who left before Junpei didn't hear it, presumably because Snake was unconscious) so se presumably left through one of the 9 doors. If she chose the one on the left she was screwed because she probably didn't have the keycard and I doubt she could solve the puzzle anyway. If she chose the one on the right She made it to the incinerator and found that 9 didn't work. At that point whether she survived or not depended on whether she could guess all the possible numbers in time, and if the possible numbers included 8.
She had 5, 4, 6, 7, and 3. 8 is possible, so I think she could get that in 9 minutes.
I guess it is improbable... she already has a 50% chance (right or left door) and that's worsened by the possibility of hearing thumping in the coffin, meeting another player, mental breakdown, and giving up at the q.