The zoom is disabled via the viewport meta tag they are using.
<meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
I don't know know why they are using that as I never use that in my sites. I recommend this method:
<meta name="viewport" content="width=device-width, initial-scale=1.0">
Of course there may be a good reason why discourse does this but as far as I am concerned you should never disable zooming unless its for a native app. When developers disable zooming they are usually more concerned about their design than they are about their users which is the wrong way around.