In fact, he's repeatedly spent this whole election patiently trying to explain to people that there are odds other than 0%, 50%, and 100%. Your remark here proves you missed that lesson entirely.
Those are good odds, but it is indeed still "extremely possible" that Republicans could win a 3 in 20 dice roll.
There's techically no such thing as "extremely possible." Either something is impossible (0%), definite (100%), or possible (anything in between). There are no degrees of possible, only degrees of likely.
By "extremely possible" he means something that, while unlikely, is likely enough that we should be willing to prepare for. As opposed to, say, the odds of winning the Powerball.