A democracy isn't defined by changing government. It's defined by a system of government by the will of the people. Generally most people agree that 'democracy' is shorthand for 'representative democracy' whereby most people can vote freely and fairly (I say mostly because even the most democratic country doesn't let children vote). There are opposition parties and regular elections in Singapore so by that standard it's a democracy.

That being said, I don't think it counts as a logical democracy. Just not for the same reason you quoted above. A single party being repeatedly elected doesn't, on its own, prove a place to be undemocratic.