I can see the confusion, as even Wikipedia talks about the river flowing through it, and indeed, some fjords are freshwater.
Certainly, at least the photos of one kayak company indicates it's river-based, primarily.
However, another literally describes sea kayaking in the fjord.
Ultimately, it's going to come down to whether or not you have an argument with your insurance company. So it's best to contact them, ask if THEY consider it sea kayaking, and regardless of the answer, get it in writing. That was you know where you stand, regardless.