The comparison doesn't make sense though. There's absolutely no reason you can't build an interface using single-page that functions to the end-user identically

The comparison doesn't make sense though. There's absolutely no reason you can't build an interface using single-page that functions to the end-user identically to any multi-page form. You can't necessarily do the reverse (multi-page to emulate single page), but that is also irrelevant to the top level -- if the SPA has 10 times the complexity to build an identical UI, then it's probably going to be harder to maintain for no value to the end user (which isn't to say no value). What I got is that they built different UIs and found one of those UIs was better than the others; that says nothing about single-page vs multi-page. Maybe it was easier for them to build the "winning" UI as a multi-page app, but I didn't see that being said.