Well considering that the author went on to work at Apple, it sort of makes sense. I don't think he was explicitly told not to continue development, but it is likely that some sort of boilerplate part of his contract restricts him.
Employers really need to stop trying to control what their employees do while off the clock.
I won't sign a contract the restricts what I do on my free time. Any place that does that is not a place for me.
If you get a chance to edit that typo I'll delete this comment to reduce clutter. :-)