Erlang and more
The article mentions three factors that (somewhat) improve code quality: Strong typing, static typing and managed memory. Erlang has strong typing and managed memory, but it is dynamically types. So by the (somewhat weak) conclusions in the paper, it is no surprise that Erlang is close to the average.
What would be interesting is factoring out the number of years the programmers have worked with their language: C and C++ programmers have at least potential to have worked longer with their language than Erlang, TypeScript and Haskell programmers, and it is very likely the case that programmers who know their language intimately will make fewer errors. This also makes a case for languages that are sufficiently simple that you can actually manage to learn all of it.
Also, these days, huge libraries mean that you quickly can produce useful code in almost any language, as long as you stay within the scope of the libraries. This makes the actual properties of the language itself largely irrelevant, again if you stay within the scope of the libraries. So a true test of language (isolated from library) productivity/quality would require giving programmers a task where they can not make any significant use of non-trivial library functions, or where you deliberately forbid use of anything but the most basic libraries, e.g., by limiting the total library code used to, say, 1000 lines.