IBMer her also. I don't know the VNIC initiative but it might be a question of contract, where all IBMers working on that specific open source project need to clearly disclose (being an IBMer) and the email is the way to do it (IBM might measure the participation).

Might also be a question of tracking a "conflict of interest", where the professional in question might work on a similar proprietary product inside the company.

IBM certainly allows you to do your side gigs, but you need to be transparent with the company about what it is and that there is no conflict with your work.