What a dumb article. Microsoft (presumably) has been actively engaging lots of developers; they don't need GDC to do so.
Microsoft have been actively engaging the established developers who work for major publishers.
Over the past generation the industry has changed dramatically in that regard and now the biggest runaway hits often come from small indie teams no one sees coming.
GungHo is a small Japanese start-up who's flagship title (Puzzles and Dragons) is quite possibly the most profitable game at this point in time.
Mojang's Minecraft is a multi-million seller on Xbox 360, where it's sold far less than it has on PC.
Unless you think MS has a crystal ball and is identifying every break out indie/small tier developer this far in advance there is a VERY real weakness by not getting to pitch the next Xbox at GDC.
What was Sony's PS4 unveil really focused on? Ease of development from a technical standpoint with a shout out to a far easier to access storefront.
So what do you think they're going to do at GDC? Likely disseminate a TON of information on just how easy it is to get on their storefront. To thousands of smaller developers who can't afford an Xbox dev kit even if MS knew who they existed and was willing to offer access.
It isn't a "disastrous mistake" like CVG is calling it, but it's leaving a pretty significant opening for Sony to further pitch their "system for developers and gamers, by developers and gamers" mantra uncontested.
Not only does that get more developers interested in Sony's platform, it likely results in more quality unaffiliated titles passing through their screening process for first party distribution.