yes, but source 2 isn't half life 3.
any exposure is good exposure. no one is going to be a hippie thinking, "we don't care we so popular #yolo".
if they are going to announce it, might as well do it through a platform where millions of people tune in.
That's why I said communicate with fans. Portal 2 was 'announced' via a patch to the Steam version of Portal 1 (modifying the ending) that fans were the first to see. If they're going to announce it it'll probably be more directly tied to their community rather than a massive conference which they've historically never done for a game announcement.
Their business allows them to have that flexibility, hell their business relies on that relationship with their fans.