That's probably why they said "it's harder to oppress people with guns, so they do it less" and didn't add "and if it is hard to oppress people that makes you effective".

Basic "necessary but not sufficient" thing here.

The counter-example would be places where armed people are kept under the boot by those in power anyway, where the government is very effective at oppression.

edit: unless you are equating "able to do things that make citizens want to shoot you" with "effective"? I'm assuming you misunderstood, which is the charitable reading.