They're not even saving any money. Syntax highlighting is a trivial workload, whereas the average SPA spends a lot of time in pointless roundtrips that have the server send more data down the pipe than the SSR equivalent.
A fun part of a retro at my company last year was me explaining to a team, “had all of your pods’ requests succeeded, the DB would have been pushing out well over 200 Gbps, which is generally reserved for top-of-rack switches.” Of course, someone else then had to translate that into “4K Blu-Rays per second,” because web devs aren’t typically familiar with networking, racks, data centers…
If github has a million users visiting it per day on a FRESH cache, and all of them have to download at least 10 megabytes of text data (both of these numbers are far too high), you are at ... 0.015 "4k blurays per second". Yeah I think MS's datacenters will survive.