tmux + vim (akin to Windows Remote Assistance) can be good enough in some situations.
Not really. I use tmux + vim on a daily basis, but it starts with having just one cursor. Goes on with not being web-based and in the end, it has no offline collaboration support (that requires some kind of merge strategy). So yes, there are some use-cases when it is sufficient for collaboration, but most certainly not for the use-cases described in the blog post.
You could easily make tmux + vim web-based if you desire (I don't see why, I'd just use Mosh or SSH instead). For merging you could abstract the content to a Git repository. For multiple cursors you could try Wemux [1].