Couldn't one say that the endings of Dragon Age Origins "conclusively tied off this protagonist" with regards to the Warden? In fact, much more conclusively than the events of Trespasser did the Inquisitor b/c the Warden had no real hanging plot threads by the end. They slayed the archdemon, and either lived to be a hero or died in the attempt. Their goal was accomplished. And yet the Warden, if alive, still returns as Warden Commander of Amaranthine to experience a whole new, mostly unconnected adventure.
The story of the Inquisition, and our role as its leader, is concluded. If you consider the protagonist to be the Inquisitor (or the Inquisition itself) then its possible to say "mission accomplished". The person behind the Inquisitor role(Trevelyan/Lavellan/Cadash/Adaar), however, has more to deal with still, as the DLC itself shows with the fervent planning in the basement that all PCs see no matter their choices.
I'm not trying to be overly literal here with this slide. I think this differentiation btwn the Inquisitor and Trevelyan/Lavellan/Cadash/Adaar is a valid interpretation b/c it would actually be keeping with the theme of how the Inquisitor (and the Herald of Andraste) was becoming a symbol that was almost a separate entity from the actual person behind it. Many companions commented on it: Varric, Cassandra, Vivienne, etc. And there was a general theme of people wearing a mask or Becoming the Mask throughout the entire game. It wouldn't surprise me if the dev team also thought of the Inquisitor in this way on a meta-level. And, due to this, its entirely possible for them to say they want to "tie up" the Inquisitor as protagonist in DA3 and still have Trevelyan/Lavellan/Cadash/Adaar playable in some capacity in DA4.
tl;dr that slide doesn't rule out being able to play as your Inquisitor at some point in DA4, imo.