With heartbleed, a bug in the implementation of the protocol led to the server randomly leaking contents of the server’s memory, which could be anything from private keys to user or system passwords to other confidential information. No passwords or MitM was required. You can read more at heartbleed.com
And it still doesn't matter, because sshd literally never has the private key that allows access. If a server only allows access via SSH key, you could literally have a complete RAM dump of the whole system and not be able to access it.
