offline web apps are different than PWA. A PWA doesn't necessarily work offline, but more is independent from the connection / loading of it. I do think most PWAs do work offline, but doesn't mean it's a requirement to call it a PWA.
Similarly, an offline capable web app is not necessarily a PWA, as PWA carries a lot of features to it besides being offline capable.
An offline web app is a frontend-only application (just HTML+CSS+JS or less) that can be loaded from any medium (internet, usb stick, direct TCP via netcat or any other transport) and work in your browser without requiring a remote connection to allow usage of it's features.
So yes, this would mean it doesn't run the risk of ex-filtration or snooping at the transport layer, as the data never leaves the specific website context in your browser.
Similarly, an offline capable web app is not necessarily a PWA, as PWA carries a lot of features to it besides being offline capable.