Think about it from the side of the creditor (the person to whom the debt is owed).
The point is, if the creditor is owed a debt, and receives payment for it, and A) He has no reason to think the payment was a mistake at the time he received it, B) He did not falsely induce the debtor to make the payment, then the creditor should not have to worry that the debtor will come to him the next day and say “Uh, that was a mistake, I want the money I owe you back.”
This is not the only class of accidental-but-plausible transactions. Why not just use "reason to believe the payment is valid" directly rather than singling out creditors?
What circumstance do you have in mind in which the person receiving the transaction would A) have reason to believe the payment is valid, B) not be considered a “creditor”?
One could have a reasonable belief that the payment was intended as a gift, in which case the recipient would not necessarily be a "creditor" but there is still no reason for them to believe that the payment was invalid.
Here's the distinction though: $400 out of the $900 million was returned. Thus, it seems like we can't get past your part A) in this case, as hedge funds had enough reason to think the payment was a mistake that they returned it. I doubt the creditors in this case are so heterogeneous such that one creditor has no reason at all to think it was a mistake while another does.
The question the law asks is whether they knew it was a mistake at the time they received it. Not whether anyone later decided to be a nice guy upon learning it was a mistake.
In this case, what happened was, Citibank sent out these payments on August 11. Each creditor received a payment in the exact amount (down to the cent) of what they would be owed if Revlon had actually intended to pay down the debt in full, with interest.
On August 12 they told all the hedge funds “oops, send it back.” Some of them said “Hey, no problem, it happens to the best of us, here you go!”, and returned the money. Some of them said “Uh, we’re not going to do that.” And this latter group are the ones who just won in court.
The point is, if the creditor is owed a debt, and receives payment for it, and A) He has no reason to think the payment was a mistake at the time he received it, B) He did not falsely induce the debtor to make the payment, then the creditor should not have to worry that the debtor will come to him the next day and say “Uh, that was a mistake, I want the money I owe you back.”