There are some reasons, called, "automatically unfair" reasons, for which you cannot fire a person: e.g. being pregnant, being a member of a union (or refusing to be), refusing to work on Sundays, and general sorts of sexual/religious/racial discrimination. Outside of that, you can dismiss somebody for any reason or no reason within the first 2 years of employment.
After that, you can dismiss a person generally only due to their incompetence, negligence, legal incapacity[1], or because you don't have any work for him to do. You are required to be fair and reasonable, a standard which is generally interpreted in the employee's interests.
One element of the article that seemed particularly off was that he claims he still doesn't know why he was fired. In the UK, you have a legal right to know the company's reason for firing you, and you can sue them if you think they lied about it.
[1] e.g. if a court order prevents the school janitor being within a mile of a school, you can fire him due to being unable to do his job.
I'm not sure this is factually correct, but if it is it'd be in direct conflict with what lawyers told me when I was forced out of one job in the UK. Essentially, I was powerless to do anything, since I hadn't been with the company for two years just like you say. When I was told I was being forced out, I immediately got in touch with two lawyers specializing in employment law, and independently of one another they said the exact same thing: prior to two years at the company they can do whatever they want, giving reason or not.
I was indeed given a reason, however I'm absolutely positive that it's not the truth. The company did right by me in the end anyway in terms of severance, so there are no hard feelings, but the truth would've been better. At least I imagine it would have been. Oh well.
He very likely knows, as he was in some way involved in this drama: http://readwrite.com/2014/03/15/github-julia-ann-horvath-har...
What his actual role was, however, is likely unprovable or not worth trying to prove. As informative as his blog post otherwise is, going into the "reasons" would likely re-open a can of worms everyone is happy to have closed.
Edit: I'm not saying he did anything untoward. I just find it highly unlikely that it's a coincidence that he's one of the names named by Julie after her departure and shortly afterwards he's fired under "mysterious circumstances" despite being one of the most productive and senior developers in the company. He just got caught up in that drama somehow, and as he says himself in the blog post "HR is not your friend".
Does it mean that workers also have to give a 6 month advance notice before quitting?
Actually the worker does not have the option to do that, since it opens them up to breach-of-contract lawsuits. They could conceivably do other things, but they'd need to be VERY VERY clever about it to not break some law.
What happens if they quit showing up for work?
It is rather rare a case.
It is common in many companies for there to be a two-way 6 month firing/quitting limititation, meaning: At the point the worker turns in their quitting notification, they can make an agreement with the company to be let go earlier, or the company can insist on them staying the full term. Similarly when letting a worker go, the company has to let them know 6 months before that, though the worker can agree to leave earlier.
Of course faster firings are possible as well, with "immediately" usually reserved for "breaking a law". Additionally workers can be let go early for performance reasons, but only after three separate, paper-trailed notifications of such issues. Additionally, those are unlikely to be trivial, since a worker can sue their company for the remaining wage, if fired unfairly, and if successful the company will have to pay all legal bills of both sides.
This means that both sides here tend to make a lot of effort to avoid firings. The company because it's just easier to help an employee improve, and the worker because they'll see it coming.
I've been in the situation myself where a company expected certain things of me, that just didn't fit my nature and health. We tried to improve things informally for a while and i kept failing. Then we had a serious talk and agreed that things wouldn't work like that, and now i contract for them, with both sides being very happy.
--
An important contrast between that, and the american system, is that i knew at every single point exactly how far away i was from being fired. Due to the wording of my contract, i was at all times, right up to the end, exactly 3 months away from being fired, at least. My livelihood was never in danger, and i was given plenty motivation to help both myself and my company, instead of being put into an adversarial situation.