Even if there was a splice, the anchor might still hold body weight. Not the splice itself, but because it was girth hitched to two other slings, the friction of those two knots would hold some weight on the single strand between the knots. Hard to say how long it would hold, but I could see it holding for a while before the free end slipped through.
If they had girth-hitched the slings to the tree, and the failure happened behind the tree, it would pull down both ends leaving nothing up top.
I'm not saying that's what happened as I've heard nothing about it.