Also, he's agreed to stay on as a consultant for the next six months, which I assume is not something you'd do if you were planning on working for a competitor at the same time.
I obviously haven't seen his contract but it would surprise me if the noncompete clock started before his final day as a consultant. If it did, that would sort of eliminate the value of having a noncompete in the first place, wouldn't it?
I left a company with a noncompete in place (it was largely valid) and did consulting work for them. The two relationships are completely separate. As a full time employee, I had signed the noncompete. As soon as my full time employment ended, the term of the noncompete began counting down. My later relationship with the company was completely separate, with its own contract (which I ensured had no non-compete clauses).
I could easily see similar happening for Jim, here. The full time role has expired, and there is a new, separate contract for consulting work.
I've done similar things in the past, but my value to those companies' competitors was much lower than Keller's would be, presumably, to someone like AMD vis-a-vis internal problems and short-term strategic thinking. The importance of enforcing a noncompete and accompanying garden leave increases pretty linearly with how senior you are in your industry, in my experience.