The main idea for the Ducks was to acquire a serviceable NHL player and clear out Kesler's LTIR contract. From the Golden Knights' perspective, the trade was a way to move out a sizable contract and provide the Golden Knights with some additional LTIR relief that would give them some extra flexibility for when it came time to activate defenseman Alec Martinez and captain Mark Stone from LTIR.
The problem was that Dadonov had a modified no-trade clause in his contract and the Ducks were on that list. In the end the trade was voided, but that didn't stop Kesler from having some fun with it on social media.
Obviously, Kesler is not taking this whole situation very seriously. His career is done, so it certainly doesn't matter to him where his contract resides.