Icelandair should do a better job on their website publishing access rules. Looking up lounge info on the FI site, I got to this link:
https://www.icelandair.com/support/airports/
But that link doesn't actually state which lounge! It seems to go to a generic SeaTac port page.
I found a blog review post (dated 2022) that says your access should be to The Club at SEA, and a photo on that blog makes me think it is correct (though I'm otherwise unfamiliar with this particular website so YMMV.)
https://worldtraveller73.com/2022/03...ttle-keflavik/