The admin oversees everything, and because the said moderator is not active today doesn't mean he or she won't be tomorrow. People have lives outside this forum. Unless there is some sort of danger attached to his account for suspected hacking or anything of that sort, I don't see a reason to denounce the person from his position… that's just my 2 cents.
Absolutely, the admin overseas everything on the forum but as human, he could still forget to handle some things that are not so urgent, such as this one. The board in question is not lacking moderator because it already have two other active moderators.
Maybe the admin didn't also forget, It's possible that this moderator must have privately communicated with the admin and have informed him about their intentions of being inactive on the forum for some months.