I did the Cursed Tribe quest for the sole purpose of looting Chief Yamarz' armor.

When we came near the final giant, he made an offer and I told him he should kill it himself.

He died instantly and after that I took care of the giant. Then I looked around for his body and... nothing.

Could the giant's attack have sent him flying beyond the cave's area, causing Yamarz body to virtually disappear from the game?

His body is always there when I've done this quest in the past. Perhaps the gave glitched on you and he disappeared. Try reloading and trying again.

But did you finish the quest the way I did?

With Yamarz stopping to talk, giving you two options and then you tell him to do it himself?