Earlier quoted context omitted.
Well if it was only 100 lines of plain JS then how would one guard against reflection attacks? I.e. submitting HTML (like script tags) then getting that to render when others view the tainted data.
Because on this way of building sites, the user submitted data is escaped before it reaches the browser. E.g.: https://go.dev/play/p/MmNSxU5QfAb (hit run to see the output). The JS wouldn't need to do any escaping, because it's not trusted to handle any unescaped data. It's operating on the already-escaped html template.
Q: "how do they use the workarounds needed to secure the more complex approaches?"
A: "those security concerns don't exist in the approach, no workaround needed. That's part of the simplicity".
It just represents a fundamental misunderstanding, but it's not their fault, they've never seen anything else. Like someone using a JWT instead of a session cookie.