To be fair, the grandparent said 'runtime errors', not 'bugs'. And it is a stated goal of the Elm language that runtime exceptions thrown from the compiled Javascript should be impossible. As far as I know, they have succeeded in that goal.
How this relates to 10x or not I don't know - I'm not sure the discussion is a productive one without further context.
It might be that for some tasks, Elm really does provide a huge benefit in productivity. In that case, it's probably fair to call a developer that selects and uses the tool correctly a 10x developer - provided they are able to evolve the culture of their company so that others can take advantage of it.
For other tasks, or in other contexts, Elm might be a hinderance. If it's lacking an indispensable library, or if the company culture is not ready to accept it as a core language, it won't help anyone to rewrite in Elm.
As always, one of the key things that makes a good (even 10x) developer is the ability to select the right tool, and to wield it gracefully.
