Can confirm the same issue, it even lists the text beneath as "Each Season, there is a new Exotic weapon earned via the Season Pass." which heavily implies this isn't intentional. My guess is at some point, they decided to swap Ice Breaker for Alethonym in the dungeon/season pass, and forgot to update all the associated tasks tied to the weapon, that would at least be a reasonable mistake. (And no, trying to use Alethonym does not progress it. This effectively softlocks anyone trying to progress until the dungeon is out, or they fix it.)