The blog linked from your MSDN article goes into more detail.

http://johndyer.name/native-fullscreen-javascript-api-plus-jquery-plugin/

Basically, IE doesn't support it now, and may not in IE 10. Other browsers support it with very recent releases, but the API was only proposed in the W3C 4 months ago. Actually using this would therefore be fairly difficult and problematic, so it's probably premature to do so. The C9 devs would have to speak up here about this topic, but I wouldn't blame them for not implementing this feature at this time.