It's not perfect. But what it means is that if you're going to impose a policy that unpopular on half the population, you may have to go as far as the Civil War did, including accepting that many of your own people dying. That's... somewhat daunting.
And a tyrant, while perfectly willing to bring their own guns to turn on citizens, would have a military that might not be willing, especially if they didn't agree with the tyrant. (The Civil War wasn't just the US army against the citizens of the south. It was the army members from the south against the army members from the north.)
It's far from perfect. It demands a tax in blood during peacetime as a faulty guard against tyranny in time of civil war (to say nothing of the fact that---as the Civil War indicates---there's no particular guarantee that the armed civilians will be defending particularly virtuous rights against government interference; it was, after all, the state's rights to nullify laws curtailing slavery that was the crux of the conflict).
And a tyrant, while perfectly willing to bring their own guns to turn on citizens, would have a military that might not be willing, especially if they didn't agree with the tyrant. (The Civil War wasn't just the US army against the citizens of the south. It was the army members from the south against the army members from the north.)