One way I interpret this, is code of conduct. I worked for a bank for a long time. You were always expected to abide by the code of conduct outside of standard hours. Generally the bank's code basically said be a good member of society. So, in the IBM context it's possible there is an agreement or code where it says they can work on software projects, but they need to identify themselves as IBM employees at all times. I'm just speculating here.
Also possible that they literally may have an agreement forbidding them from participating in software projects, or that IBM has the rights to all IP that they might think up on their own time while on salary.
I remember one agreement many years ago when I worked in IT, that said the company had the rights to whatever I thought up while working for them and it didn't specify that it had to be directly related to their area of business. So if I wrote a sequel to Gone with the Wind, they would have had the rights to it.