Is it possible to lose Catan to a player with fewer points than you?
Yes, as it says in the rules you quoted you can only win on your turn

Victory Conditions: Can I win during the special building phase?
Answer: No. You can only win during your own turn.

So yes if you reach 11 points and it is not your turn it doesn't matter and someone else will still win if they reach 10 points on their turn and it is before your next turn. In Catan it isn't just about reaching 10 points to win but to do it on your turn as there is a chance you could lose some points before your next turn.