Independently on how treacherous it is to attack during a "peace conference", the move is valid. Hell, if I happened to be leading an army, I woulnd't line up my guys nice and comfy to get shot down by the enemies. If you choose to follow the rules imposed by your enemies, you're predictable.
Of course, the best course of action is always to make your enemies believe that you follow those rules, even if you don't. Then you can use an opportunity such as, say, a peace conference, to turn the war in your favor.
Not doing everything you can is a crime against your side.

By the way, if Parson has the smarts, he can order sizemore to open a pit (like the one he opened for attacking with golems earlier), make Bogroll and Ansom fall there, and close it. Result: Bogroll and Ansom live, and the leader of the coalition is captured.

Captured Ansom is way more profitable for Parson then croaked Ansom, even more profitable then raised Ansom.