As you suggested I used the chat feature, and the explanation was something I have never heard of before on Viking Ocean. That included tour does indeed finish after the ship leaves port! Supposedly, a shuttle bus will take you to the next port.
In this case, the ship leaves Seydisfjordur at 4 PM. A shuttle bus takes the excursion group to the next port (Akureyri where the ship arrives 8 AM next day!). This seems very weird and I didn't get into the logistics of where you sleep, eat, etc., and just canceled the excursion. We can do a port walk easily enough on our own earlier in the day if we want to.