I feel like the author makes a good point, but it does not solve the original problem he was describing. The only reason why those websites disappeared is because the owner was no longer interested in maintaining it. It's a human factor, not a technological one. Even if they had written their files in markdown and git, the website would have disappeared anyway if they stopped paying the domain/hosting fees.

Now if you're arguing for putting the website source on GitHub, that's an entirely different matter. GitHub addresses the human factor by being entirely free with no effort required for site upkeep. That's why it's durable, it's not about keeping it as git and markdown.