The VPN app would send every information possible about the user to Facebook. One of the things it did was to be a man-in-the-middle that decrypted the internet traffic to and from Snapchat so that Meta could collect analytics on how people were using Snapchat.
Meta is such a piece of shit company.
In this case “national security” means ensuring US intelligence affiliated companies have full access to control over everything people do with internet connected devices.