this is just limiting what apps can be opened, and it only works for flatpak apps. how will you disable all the other apps that are installed? how do you disable the shell which could be used to download a non-flatpak browser?

and as you said it does not even try to limit which websites are allowed to be visited, or for how much time can the computer be used. a pihole can be circumvented with DoH, for which there is an easy toggle in firefox, probably chrome too