Half of those example sites he says should be SPAs should not be SPAs at all. They can be, and are, but certainly don't need to be by any stretch.
Yea. I didn’t understand the criteria given: long running sessions and authenticated content. Why do those necessitate a SPA?!
Authenticated content is likely for a mix of reasons. It can't be indexed, so doesn't need to optimize for SEO. Users that log in are higher intention, so they're less likely to bounce if the page loads slowly.
It's definitely more complex than just these things, but the inverse (an app that requires SEO and minimal upfront costs) are often used as justifications for moving away from modern frameworks.