I don’t think EAs are focused on assigning credit here. It’s very consequentialist; they would say “it doesn’t matter who saved the 100 lives, it matters that they were saved.”
It matters if the 100 lives are suddenly 200 lives when you sum them up. It matters for decision making. It matters for questions like "should I rather do it myself or start a foundation to convince as many other people as possible to do it".