I am going to guess they are getting a massive surge of traffic about now and yet the site still renders in a fraction of the time it takes most "modern" web apps or even just static content sites - this is how it should be.
The page was also framework after framework piled on top of each other, so they’d be using React and jQuery at the same time, stuff like that. And so marketing could change whatever they wanted whenever they wanted without breaking the entire site, everything was served from a CMS.
So it’s probably a lot of that.
At a previous gig we had a similar situation. It was so bad my coworkers and I would joke that we needed one of those "break in case of emergency" boxes with the glass front and a hammer hanging adjacent. Inside the box: a can of lighter fluid and a match. The purpose: when the crazy got to be too much you just light own hair on fire and then go run around in traffic.
What you just described would definitely be a break the glass kinda situation :-)
Before going static as well I made a WP theme with Twig, removing most <?php wp_magic_function(); ?> from there. It was fast, way faster, than the usual WP themes. Obviously nowhere close to a static HTML.
Thats probably it. Some beginner has written an n+1 query and the page is slow as shit now.