I think you're missing the point. It's not written in Python. It's written i RPython, which is vastly different. Technology behind this and PyPy is the same, however there is no Python usage here.
Additionally, erlang has avoided scaling issues by being functional. This might be good or bad, depending on your viewpoint, but just implementing a ruby interpreter in erlang won't cut, because you're not answering any of the hard questions about the state. You might answer "meh, ruby is just a broken language", but well, this has nothing to do with that article.
As RPython is a subset of Python, that statement is technically correct but misleading. Stating that Topaz is "Ruby written in Python" leads to the idea that it's written in / runs on CPython, which is really not the case.
Additionally, erlang has avoided scaling issues by being functional. This might be good or bad, depending on your viewpoint, but just implementing a ruby interpreter in erlang won't cut, because you're not answering any of the hard questions about the state. You might answer "meh, ruby is just a broken language", but well, this has nothing to do with that article.