They assume existence of donors already willing to donate, there is no call for extra donations. It's just about how to do more efficient matching. In this cas

They assume existence of donors already willing to donate, there is no call for extra donations. It's just about how to do more efficient matching. In this case "altruistic donor" could mean, for example, a mother with a sick kid, which she wants to save by donating her kidney, but it doesn't help because they are not a good match. So she just gives her kidney to some random kid, with a hope that others in a similar situations would do the same, which would eventually lead to her kid getting a matching kidney. Altruistic, because it's not guaranteed that her kid will get the right kidney in time (only that there will be some donated kidney left at the end). But if the "chain of trust" keeps going on, chances are high everybody would get the matching kidney.