I agree with the shade bit, however, I've personally seen several high visibility execs 'fired' with this exact line about staying on for 6 months for the transition. I've never seen anyone stay longer than 4 weeks.
If I'm being critical of anyone, it's the person who wrote the press release and decided on the "effective immediately" and same date, not Keller. They have to be aware of the optics of it.
I am thinking if it was an opportunistic PR. They knew he was resigning for some time, but didn't want some bad PR from it as Keller was known as the man to save Intel. Given the market situation today they have decided to rush into this announcement.
And yes I do agree with your take on it. Intel has some of the best PR and marketing folks in tech industry. The writing is surely something to be worth looking into.