The article is silent about the terms of the severance. They are only telling us the consulting requirement. What is the severance package? If that package is worthless, then it would be easy to walk away from the whole deal. If the severance package is considerable, then they will be paid, in effect, for their consulting services.
That's the question to ask. This sort of service isn't free and it would be fantastically stupid (and risky) to demand it.
I was working for a company that was acquired by a rival. Everyone but a select group was shown the door; those of us who were lucky were put on a two-year transition team with the understanding that at two years plus one day, we were done.
After the less lucky were out the door, the other shoe dropped: full salary for those two years plus a year's worth of salary as a bonus. We just had to see that nothing bad happened during the transition. It was smart insurance and a bunch of us got rehired anyway.