Sorry, you're right.
It seems that the ssh-agent plugin only supports stored user, passphrases, public key credentials added in the credential management area in Jenkins. Take a look at unit test , which verifies that ssh-agent is working correctly based on the public key. It is unlikely that the plugin has unverified functionality to support user authentication + password.
If you can, set the switch to public key authentication. If for some reason you cannot switch ... you CAN install sshpass in your Jenkins box, but this is usually considered bad practice.
node { stage 'Does sshpass work?' sh 'sshpass -p \'password\' ssh user@host "ls; hostname; whois google.com;"' }
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