I still see it as a Duel. It wasn't like a backstabbing, or Adolin ordering his soldiers to slit some throats.
It was a 1 v 1 of a brutal nature. yea, it wasnt as refined as a Shard duel, but it was still a fair 1 v 1.
I'm more interested in the "Adolin stood for a moment, staring Sadeas in the eyes, and then something finally snapped." line from the start of the fight. This was what made me originally think of Skybreakers, and their extra ability. But we have WoB that is not the case.
I like the use of the word Snap, just like in Mistborn and all Cosmere books, you need to be broken to make room for the Investure.