He's just restating (and spinning) a critical founding principal of the Bundesrepublik, that is the concept of a 'wehrhafte Demokratie.' (roughly - vigilant or capable of self defense) It's what underpins the ban on extremist political groups, whether left or right and limitations on free speech. It was also a guiding principal behind the restrictions on civil liberties in the face of left wing extremism in the late sixties and the seventies, e.g. the Berufsverbot.  The concept was shared by both the left and the right in West Germany.

As to your question on the fall of Weimar - multicausal and much too complicated to deal with in a blog comment. However, it certainly was a consensus belief that Weimar did fall at least in part due to a refusal to crack down hard against anti-democratic elements that used democratic freedoms to destroy democracy and freedom.