He has account on other sites as well, but he chose to hide some of them from the network profile, resulting in two things:
- On each "hidden" site, we see only the account on that site.
- No link to network profile on those sites.
This makes finding the associated accounts difficult, but not impossible.
I prefer not to prove my claim by posting a link to respect the user's wish, but believe me that he has other accounts.
Another option, is a deleted account. When a user with 200+ reputation (meaning the bonus is already awarded) deletes his/her account (which triggered the bonus), the bonus is not removed from the other sites, which can also lead so a case where user without any account with more than 200 repuation has the association bonus: Association Bonus not removed upon account deletion