Depends what you mean by debunked. It is a bit of a fantastical idea to begin with requiring technology and engineering on a scale much larger than anything in human experience. It is, therefore, a bit difficult to come up with a plausible plan let alone debunk that plan. I, for one, do not see any fundamental reasons why it couldn't be done, but I can come up with plenty of practical reasons why it probably won't be done anytime soon.
I thought the lack of a magnetic field was considered to be such a "fundamental reason", i.e. without one away any significant atmosphere that's created will be stripped away by the solar wind.
The issue is timescale. Yes, the atmosphere is (very slowly) stripped away by the solar wind, so we could not simply terraform it and forget about it. On the other hand, the timescale is very long by human standards, so as long as we produced new atmosphere at a rate faster than it is being stripped away, I don't see it being a fundamental problem. I could be wrong, of course.
With a weak magnetic field, wouldn't something such as a solar flare, which the Earth has more sufficient protection against, possibly wipe out any part of the surface it touched on Mars?
