Is it not the EP's responsibility to determine how update planning and scheduling happens? They may not be directly involved in the story planning, but setting up the thematic update planning we've seen with EGW and now Legacy of Zamorak would seem to be the "big picture" stuff I'd assumed the EP decides (with input for you and other team leaders of course).
It does depend a lot on company and department. When I worked at Rockstar, the EP at the time (Leslie Benzies) was functionally the creative director and lead designer as well, which has good sides and bad sides. On RuneScape, which is a huge, complex, multi-faceted game with a lot going on beyond the content itself, the EP has to wear a lot of different hats and deal with a lot of different people, both above and below them. In the specific case being referenced here, of the EGWD and the way story plugs into it, none of that really involved Mod Warden at all.
The general strategy to plug story into all or at least most updates was mine, along with the more specific story elements of delivering the elder gods in the form of a god wars dungeon, splitting that dungeon into four updates, etc.