Long running sessions seems pretty straightforward. The cost of an SPA is heavily front-loaded, so the longer you have the app open, the more that cost can be spread.
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.
Not needing to be indexed is what allows SPA, it doesn't justify it.
Similarly, page speed should matter regardless and tbh what I've seen in the wild people restricting themselves to serverside renders have far superior load times due to cachability and lightweight page designs.
I think the article hits the nail on the head, were opting for devex over UX. I don't think it's really a problem, I use react in a large org (one of the spa examples funnily enough) and it does enable workflows and org structures that would not work without the site being a spa.
Whether that trade off is worth it I'm not sure. While it enables org structures that work for us Im not convinced the resulting empire building and feature spam was worth it for users.