Not exactly. A "hot" wallet is one where keys are kept on a computer connected to the internet, and the risks associated with that. If you have your posting key in Hive Keychain, it's still possible a hacker could get you to, for example, upvote something that you don't want to, or make a spam comment you don't intend to, but they couldn't get you to transfer money. Putting your active key into keychain makes it more "hot", because then transferring some of your liquid Hive or HBD would be possible if they managed to compromise that computer.
Memo key has no authority on the network, but can be used to sign and encrypt messages. This allows you to prove who you are without using a key that could be used for transactions if compromised.