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.