A few reasons:
1. If you were fired you would look silly to everyone who knew you were fired by keeping that up there.
He's going to look silly when he takes it down. Especially since that's the exact place where he was busted, where people are hitting F5, and where he ran away from. I'm sure he's going to wait for some heat to die down and then muster some liquid courage before going back.
2. If I were fired why would I want to pretend I'm still working there. I'd be po'd and remove it ASAP.
It's not about "pretending he still works there", it's just one of the unpleasant things he needs to deal with.
3. If I'm Microsoft and I just fired someone for breaking am nda or whatever, I believe my legal department would be ensuring he removes any public associations with the company.
If companies had time-sensitive detailed instructions on what specific activities to perform on social media sites (after being fired, even), then guys like this wouldn't be destroying their careers.
To get to where he is in the company probably has some allies in the management team, so my guess is that he just got into trouble but is still working there.
So you don't believe that videogame journalists have sources close to the situation, who report that he's been fired... because you don't believe that executives
get fired for public meltdowns?