I can't remember ever seeing one of the fundamental reasons explicitly being called out..leading to vast amounts of devs never truly grasping the why of 'why are SPAs faster'.
(e.g. me opening GMail in Indonesia and never being able to load it)
I dislike the general SPAs are faster "truth". They might, they might not and the initial load can be just too much. You can sure spend time to optimize SPA to perform well though. Btw Turbolinks tries to solve this "do not parse JS again" thing and it's where the speed comes from.
The article was about not doing a SPA, so I was confused.
Yes, they found a way to extract the part that makes SPAs fast and applied it to their legacy code-base.
My idea of SPAs performance gain was mainly that they didn't send HTML, but JSON, which I cosindered vastly smaller.