I guess they mean launching webapps with --app=, which launces the page in a chromeless window.

Firefox recently learned --ssb (after you enable that in about:config), but it's use is much more limited, geared towards kiosks. Things like right mouse button and extensions are disabled, and you cannot open external links.

Chrome has something called Desktop PWA that allows a PWA (that meets some additional requirements) to be installed as a native app. On macos it has its own .app bundle that can be launched by user and participates in the app switching as a separate app, but underneath shares process tree as the user's chrome instance.

So it's like having an Electron app with stricter sandboxing, less memory usage, and an easier installation / upgrade process.