I'm assuming they're talking about a standard SPA (or even a conventional full-page-reload web app), or React Native or Electron on the desktop or similar. In the former case, a PWA can use service workers and caching and so on to get better network performance. In the latter case, they can leverage a preexisting Chromium/other browser instance to get most of the advantages of "write once, run everywhere" without bundling a whole browser instance with your app. I can't wait for full PWA support in Firefox, especially on desktop!