Then I was partly right, because it seemed to me that he had a different badge, and I'm pretty sure that one of those old and experienced members had a Bitcoin-Qt core developer badge.

I am not aware that such a contest has ever existed, and the only logical explanation is that the admin made it as a small recognition for some members who deserve it for a certain contribution in the development of Bitcoin.

The link to their list is in Forum ranks/positions/badges under Badges.