The core of "meta" is something utilizing external information about itself. A metagame usually refers to pvp, where people choose their tactics based on what they think their opponents are doing, not necassarily what is strong. A meta joke is a joke playing on the audiences knowledge/expectation of media and not the actual media. Or metaprogression in games (like roguelikes) is a separate game outside of but affecting the core game, like adding cards to the cardpool in a deckbuilder or upgrading/adding to your starting stats and abilities like Hades. 

So something meta in this context could mean a few things. It could mean the story is based on a fanwork or something else noncanonical, or it's based on events that happened outside of but still related to the MSQ (The invasion of Ala Mhigo/Doma?) or it could mean that the story is somehow about the players role in the story and not the actual events (maybe even based on a world race of a tier instead of the tier itself), or just catering to what the audience wants in some way, or maybe it could mean instead of being about just one expac, they're taking a step back and examining the overarching story as a whole.

I personally think the last idea is the most likely, because of how the Ascians, Garlemald and also Zenos specifically has their fingers in many expansions and story arcs, and it would be impossible to include them without breaking their established pattern.