IPFS’ DHT find content by looking up for their hash. To figure out which peer to ask for, peers need an identity of the same format. So the peerID are also multihash of the same size.
Then, IPNS records are also published on the DHT, so it make sense to use the peerID for routing, AKA the hash of the public key.
About the peers waiting for 16 peers to resolve an adress: this is no longer true, or won’t be true soon as all IPNS records will be signed by the published, so the receiving end is able to verify and use it instantly.