WHAT DO YOU MEAN THEY TOOK TWO WARDENS AS PRISONERS?!? And what happened afterwards?!?
Kamado saw them as an imediate threat. Ingo and Iscan arrived looking to check on the people of Jubilife and Akari (having not seen either leader as they were both busy doing damage control with the other wardens who are in no condition to be doing much of anything)
Kamado has more or less seen the wardens as a threat since before they first landed in Hisui and has been making contingencies ever since. When they confronted him upon hearing about what had happened he put those contingencies into action upon them.
The cuffs put upon them having a mix of different types of symbols meant to seel away ghosts. Symbols both Hisuian and other (it seems these sorts of things canonically work in the pokemon world at least to some degree) This leaves Ingo and Iscan unable to just phase out of their shackles, and unable to break them as they are welded shut not just locked.
Even then Iscan is more on his own in any attempts to escape. Ingo being more ghost than he should be he is more negatively impacted by the shackles than Iscan. He's rendered practically paralyzed.
Kamado has no intention of keeping them forever or killing them, his only goal is to neutralize the perceived threat they pose to his people.