The actual line (at 10:34) seems to be:- you can be half way in the game and start the expansion
He doesn't specifically state 'half way', but I would imagine trying to play the expansion at level 20 would be quite a challenge. A higher level will likely be required.. . .If you've already played The Witcher, and you're part way through, you'll be able to get a new quest, where you'll be able to go and take part in the new expansion. . .