Done by which side? I would not think it is a good message for Apple that they had to fire someone in such circumstances.
But yes, being loose with trade secrets like this, will most definitely get you in trouble.
Videos released bu journalists in a more or less controlled manner is one thing, industrial espionage kind of videos are very different, IMO.
Like I said, it may not have been orchestrated, but we can be very sure the PR department is making the most of it. By blowing it up like this you both warn other employees to be careful and, more importantly, get some nice media attention. Apple is very good at constructing hype. Though it wouldn't surprise me if it were to be fully constructed either. The timing is absolutely perfect, with the phone already being revealed, but not quite yet launched.
We should remember that an employee lost an about to be released phone is a bar a couple of years ago, which sounds a lot like this incident. It's also no secret that the response for the iPhone 8 has been lukewarm.
And, let's be real: nothing really exciting got revealed here, just a quick look at some minor features. Adding a bit of scandal to a minor reveal might just grab that extra bit of attention.