Doesn't matter if you can redo with a couple of jQuery lines, their site has more traffic than any other image hosting the world and they need some code that if tomorrow all the engineers leave it still can be maintained, and React plus jQuery is a extremely smart decision for that goal.
One of the "good engineering practices" is to use a baseline code widely know and that's what full frameworks like React and Angular buys you, of course you still can create a mess with any of those 2 but is way harder to do that than to do it with in-house custom framework, nothing makes an JS developer run faster from a job position than being told they are using some custom framework that has been growing organically for years, regardless of how good your documentation is.
That's exactly what happened here, and what this entire post is discussing. The frameworks aren't the problem, aren't really necessary, and there are faster alternatives if they must be used (like Preact and Svelte).
No is not what happened here, what happened here is that engineers voices get amplified from their anger because they value megabytes because they are more aware of their existence than the average joe; for average people nothing of this matters as the assets are being cached, it doesn't matter even for the developers of the site because is doing pretty well being one of the most visited sites in the world.
Again, this page for a mobile site took more than 40 seconds to load a single image. The average user is not waiting, they've already left. You seem to be missing that detail. Performance matters and has been proven by metrics and research from every major internet company.
Site popularity has nothing to do with UX. Imgur is popular because of reddit, and there's plenty of users who dislike Reddit's slow and heavy redesign too.
