After thinking about it, I wonder if it's something like the mods pulled the assignment, realising that it was a placeholder, but the recipient hasn't known that, thought the treat was their main assignment and maybe complained that it didn't include all of their requested characters?

Or there's been some confusion over whether or not the one story left on this person's account was by their assigned writer?

In other words, it could simply be a mess up on the mods' end because they haven't - correctly - identified this story as a treat rather than the fulfillment of an assignment. (But then I've never run an exchange on AO3, so I don't know what exactly any mod panels show).

Regardless of how it's come about, I'm with Isis - I think you should rebut the mods' e-mail (although as someone who hates confrontation, I realise that this may not be anywhere near as easy as it sounds).