Several of us on the Community team handle those flags. Unlike a regular site, Area 51 isn't often hit with true spam.1 Therefore, flags are not quite so urgent on Area 51 as they are on other sites. The basic timeline on the post is:
Mar 5 '13 at 3:2 answered
Jun 27 '13 at 23:24 spam flag
Jun 30 '13 at 3:01 flag cleared
The "person" who cleared the flag was the Community user, which is what happens after 48 hours if the spam thresholds are not reached. I don't know if anyone saw the flag2 or if it just wasn't seen the next two days (Friday and Saturday). In any case, the flag expired before enough people flagged it for spam or a community manager deleted it.3
In any case, the post was deleted by votes from 3 non-moderators today.
We have turned off automatic expiration for spam/offensive flags on Area 51, so this shouldn't happen again in the future.
Arguably the post you link to isn't spam in the traditional sense. Certainly the author mentioned their affiliation with the site they linked to and they certainly intended their answer to be relevant to the question.
Which amounts to a sort of pocket veto.
The question has been viewed 197 times, including any attention that has been drawn to it today.