So is this a matter of people turning mostly-static websites into React monstrosities or is it something else?

Yep, replaced simple HTML with JSON and client side templating, realised it was inherently slower so re-invented server-side generation (now called SSR, server-side rendering, because everything needs a fancy name), and then merge it all together on the client (rehydration).

All this for content that is 99% static and doesn't need that level of interactivity, even the linked site is doing it for some reason, and they don't even have comments or something that would explain it. They're using it purely for navigation where a plain link would suffice.