Edit: nevermind https://www.getmonero.org/resources/moneropedia/ringsignatur... cleared it up.

Zero-knowledge proofs/cryptographic accumulators are used to verify each coin is spent at most once. Any of 11 coins could have been spent, each owned by a different single key.

For some reason, I thought Monero was basically ZCash plus using ring signatures to make traffic analysis much more difficult even if the zk proof system were broken. I was completely mistaken.

Edit 2: Sorry droffel, I wasn't fast enough editing away my old understanding of how it worked and asking what I was missing. Thanks for the explanation.