I'm inactive today; that does not mean I will remain inactive forever. If the board in question was lacking moderators to handle the board, then the admin could have appointed others, but in this case there are still two who are very active, just as you have noted.
They appointed more than 2 moderators so they can be very active in there duty and not make the work difficult for them and so if they appointed 3 moderator and one has not been active in his duty for 1 year there's need to change him and appoint another person in other to make the job easier for the remaining 2 moderators.
This forum is a very reputable place and should not permit a moderator being inactive for a year because if this is permitted it will create some loop holes in the forum.