I believe you, but I have so many questions. Where the devs prevented from chatting with each other before starting to hammer on the keyboard? Were the backend devs not curious when they had to import a new dependency (jquery in this case)? Who did the code review?
Having a backend dev do frontend is not different from a junior employee starting at the company. People have to support them, with advice, tips and reviews. Did the company failed at regular onboarding this bad too?
Honestly the whole thing sounds like malicious compliance. People didn't want to do a thing and they made sure it will be done badly.