In this case, the post notice was applied before he identified himself as a designer. "I'm a designer for that game" is a valid source citation, and now that he's added it the post notice has been removed. It's really as simple as that... Prior to that, his simple "Only one!" answer was treated as a low quality answer by the community like any other would be.
And before someone says it, no, putting your affiliation in your profile isn't enough; posts should stand on their own. (Plus, I don't read through people's profiles to try to determine the validity of their posts). Cite your reference, if it's yourself because you're a designer that's fine.
Though do also keep in mind Do Authors Overrule Users?; just because a designer asserts something doesn't mean that the community or OP is constrained to accept it as the "right" answer. But yes, designing on that game counts as a valid source citation (backing up with game text makes it better...).