Blocks only websites not apps
Why does Quiet only blocks websites in Safari?
Due to technical limitations set by Apple it is for app developers not possible to block websites system wide with a Safari Extension. It is possible for a VPN app to block websites system wide but in that case you give all your web traffic to that service. I do not want to know anything about your web traffic. With a Safari Extension like Quiet the developer doesn't see any web traffic. That's why blocking social media is limited to Safari only.
How can I block social media apps?
In iOS 12 there is a function called Screen Time which shows how much time you spend in certain apps. It also allows you to limit your time or block apps all together.
The Verge has a nice article on how you can limit/block apps in Screen Time.