None of that requires 5Mb of cruft or an entire React app to load. It would take a few lines of JS for some dynamic comments. This is the very definition of overengineered.
Compare that to Stackoverflow which generates completely dynamic pages with more interactivity in 50 ms. Also making the site faster would generate more revenue as more users would actually finish loading page.
"Some dinamic comments" is easy to see you are not understanding all it does here, the comments are more than 100 per image, replies are hidden until clicked, the points of each comment update in (almost) real time, the comments are posted using Ajax to never lost the scroll position, same than all the other interaction such as upvoting, downvoting and report, like everything is over Ajax it needs to keep track of url history itself, all this needs to work on IE10 and other shady browsers (due being one the most popular sites). They don't have much control over some of the assets because they are from the advertiser and have little say what's going on there if they want to win money. Is one of the top 20 sites visited in the United States and I seriously don't think being slightly faster would help much or maybe at all.
And most important than all that: All the assets are properly cached, so despite bothering thousands of engineers for most people the load nuisances only happen once or at most a few times.
We're talking about just the frontend here. I can definitely redo it with substantially less code and no frameworks, including automatic transpiling for different browsers. No need for React + jQuery with megabytes of JS, but if you must have a framework then use Preact or Svelte.
Ads are different but I work in adtech and site speed matters. Faster sites make more money. Remember this is on a mobile page. People don't wait more than 3 seconds for a site to load. You're not getting any ad impressions from visitors who never show up or leave.
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.
Compare that to Stackoverflow which generates completely dynamic pages with more interactivity in 50 ms. Also making the site faster would generate more revenue as more users would actually finish loading page.