This platform is different.
1) The web app is loaded client side, doesn't touch the server
2) The nsec is handled by this github page https://github.com/44Billion/44b-vault, loaded on an iframe, that runs exactly the same open-source code on the repo. 44billion.net has no direct access to the nsec. It lives as a passkey on the device's secure element.
Soon uses will be able to switch to their own 44b-vault fork.
