It has nothing to do with you being a PR. Any visitor can ask for a longer extension.That is correct. But being a permanent resident , you can ask for a longer extension for your parents than a typical 6 month extension that is granted to most tourists. They typically approve a 1.5 year extension if you ask explicitly and provide sufficient information like insurance , income etc...
I do agree with your statement that using the word "Supervisa" is incorrect. It's just a long tourist visa extension.
It is not a tourist visa extension. It is a visitor status extension.