I agree with this, but that seems to be a problem with Github's frontend code, not their server code. I don't think that detracts from this blog post's message.
Admittedly, it's been a long time since I looked at Rails code, and I don't have the slightest idea how GitHub is actually architected. But I don't remember the "front end code" in Rails being a separate thing from the server code, typically: the whole point was server-side rendering. Is GitHub using one of the JS/TS frameworks for the front end? I do remember that being a developing trend, a few years back.
Point is, there's nothing about Rails in particular that would prevent fixing these issues, that's probably the result of development and business priorities, legacy code, etc. that would be an issue with any tech stack.