I do think it was very impressive in terms of functionality especially without relying on JS, possibly the most powerful and well crafted UI on the web, but still it was deeply flawed from a UX standpoint.
They lacked many critical features for viewing source code, like linking function definitions, or expanding directories without relying on a load.
I’m sure that the GitHub front end team was of the opinion that users should just clone every repo they interact with and use their own editors instead of the GitHub website. However this has always been a bit of a hassle compared to using the website, so I very rarely find myself doing this.