Setting up to do it outside of work as a "hobby" might still be being a good open source citizen, but it doesn't mean that IBM may not have some legitimate concerns. If there's e.g. a bad interpersonal situation on the team, IBM might not want him to interact with other members of that group anymore. Or, since he's likely handled proprietary IBM information relating to that hardware, ongoing work on the subsystem outside of an official capacity could be problematic.