I think there is a mistake here on public keys

Public keys are only used for encryption. To decrypt something encrypted with a public key you need to have/use the private key pair of that public key. Also, the key you download from AWS after they create a pair for you, is the private key. The public key from that pair is actually installed on the instance in the correct location with the correct permissions and referenced by the correct config file to allow authentication.

I am missing something or the course needs to be corrected here?