I strongly encourage people to read some books on negotiations - as well as read up on legal ramifications to some negotiations.
Pretty much all books/courses on negotiations say: Ultimatums have their place, but are a minefield (i.e. they can blow up on you), and should be used as a last resort. From a negotiations standpoint, the response was adequate - which is why they all caution against using such an approach.
As for the 2 week thing, this is a convention, but not a requirement. In my company, it's not unusual for someone to be shown the door the same day they announce they plan to leave to another company (it's not the norm, but not at all unusual). The manager/company always ponders whether there are risks in keeping the employee for a few more weeks vs the gains, and this is the question Jeff pondered - that he did this is quite normal. Will the employee provide anything useful to us in those two weeks (e.g. handoff work to others, etc)? Could he/she cause problems (bad mouth people to fellow employees, steal IP, etc). If it's a disgruntled employee, they are usually shown the door the same day. In Timnit's case, it's unlikely there was any value in letting her stay for 2 more weeks.
I once intended to leave the company I was working for. The night before, I took out everything of (personal) value from my cubicle, as well as from my work machine. Only then did I have the discussion with my manager.
Having seen how she communicates and handles difficult situations, I think she really should read those kinds of books. Sometimes her behaviors are textbook examples of what not to do.
(Hint: If you're trying to influence someone, or a whole industry, you are negotiating, whether you choose to think of it that way or not).
