There's a huge Old God buried in Ulduar that we killed a few years ago, and as established in the comic (for better or worse), dead Old Gods can still influence and communicate with their servants. Also, that wasn't Mimiron, it was a generic custodian whose entire job/purpose for existing is paperwork and being a tour guide. He has no combat skills, while there is an archmage and a heavily-armed problem-solver right behind him who handle the demon problem to begin with.
Now, whether I think it's foreshadowing is another matter entirely. I do think they're building up to another expansion themed around the Old Gods, one done with a little more finesse than their ultimately-tertiary role in Cataclysm's plot.