Mature democracies don't have coups. That problem is solved.
Democracies don't change human nature, there are still powerful people who want to execute coups in any given society.
There are still far more people in society who don't want a coup, and the desire for a coup isn't binary - democracy changes human behavior, obviously.
The topic was "Civilian control [of, presumably, the military]". The root comment seemed comfortable with the idea that individuals can go rogue if they have access to a large number of machines that unquestioningly obey them. I'm not sure why you think society being organised democratically makes that suddenly a non-issue again, and if you believe that then why did you suggest this topic in the first place?
Military generals are just people and power reliably corrupts. There is a bit of a screen to try and pick emotionally stable and patriotic ones - and I assume that generally works - but the big control is that fact that all the people reporting to them are voters. If the people reporting to them are from a smaller pool of voters and vastly more productive with machine aid, there is room to speculate that control will be less effective or maybe even fail.