Sandboxing in the browser is different than the sandboxing done by flatpak.
You really want your browser sandbox to be rock solid as any weakness allows for drive by attacks by websites.
Flatpak uses bubblewrap under the hood so it is just as secure. Flatpak also has a security tracker and when security issues are found they are taken very seriously.
I know you are saying this is a bad thing but as an American I have no issues with it.