Earlier quoted context omitted.
You can never stop bad ideas from spreading, but you can decide if you'll ever hear about them or have the opportunity to fight them. If the most repeated idea wins but bad ideas are forced to spread in hidden places without refute then bad ideas are the only ones getting any voice and they win by default.
then bad ideas are the only ones getting any voice That's irrational, resting on the assumption that good ideas which encounter opposition in public discourse thereby cease to exist.
This is just as true of good ideas as it is of bad ones, but that's far less of a problem because groups of people spreading good ideas among themselves isn't typically dangerous, although there are a lot of advantages to having diverse perspectives in every group.