If I'm not mistaken, Microsoft is a big customer of IDC. While IDC isn't strictly pay-to-play, many of these "independent" analysts essentially make money recommending products produced by their customers. I'd consider it likely that Microsoft's cash is a factor in IDC's coverage.
Also worth noting is that IDC's track record here is pretty bad. From the article you linked:
"IDC predicted that Windows Phone would become the number two smartphone OS in terms of shipments back in March 2011"
Well, for one thing, they are very nearly talking about two completely different things.
This is a commentary on a statement by Nokia that, if accurate, sums up to: Things are bad and worse than expected.
The IDC press release, which lacks any information on data or methods, seems to say: If Nokia can turn its emerging market dumb phone users into Windows smart phone users, it will displace iOS as the number two smart phone OS in five years.
http://windowsphonegeek.com/news/IDC-Latest-Report-Windows-P...
Everyone's an expert.