a) the achievement (Giant Lover) is retroactive: if you already have the 11 icons, you'll get the achievement.
b) the quest is not retroactive: if you want the rewards for the 'make an icon' quest, you'll have to make another icon.
fyi: there is an achievement for collecting 151 emblems. If you have all 11 icons, another set of all 11 emblems, plus complete the new quest, you'll still be short of that achievement.
Eh, there are so many ways to achieve this quest with little effort that this is a silly idea. I am definitely not for them dedicating any resources to this, as there are better things to work on, like housing!
If there is a player who has accomplished Crest Connoisseur, maybe they can have it awarded. Quests don't have to be something that can be completed in a few days. Remember, Glitch needs to be a game that people will continue to play. Having quests that take more than a few minutes crafting make that happen.
They complain they are bored and want more quests. They get more quests and they complain they shouldn't have to do them.