Sandboxing is part of an overall security solution and also has other uses besides security. And as you know there are many other parts to securing your system. Of course if you already implement sandboxing yourself then you probably don't need to use Flatpak to make security guarantees.

I think you have made a lot of wild assumptions from reading those posts you linked, and those assumptions don't apply to everyone. Regardless of your feelings on Flatpak/Wayland in certain applications, they are being used for security purposes right now. The Chromium security model also makes a lot of assumptions and only applies to certain applications.