"Protected" is an odd word to choose here because session keys are agreed long before we know who we're talking to.
The approach of SSH (like modern TLS) is to create a secure channel between two participants and only then authenticate one or both participants by binding credentials to this secure channel.
The article gets that upside down, which is understandable because most people seem to imagine that it'd be essential to figure out who you're talking to first and only then encrypt things, but actually the opposite is better.
If you do Trust On First Use as many SSH users do, then you're correct that bad guys can interpose on that first connection if they happen to get lucky - but that's because they can authenticate as the "correct" server by presenting their own public key since you have no idea what the correct one looks like.
The approach of SSH (like modern TLS) is to create a secure channel between two participants and only then authenticate one or both participants by binding credentials to this secure channel.
The article gets that upside down, which is understandable because most people seem to imagine that it'd be essential to figure out who you're talking to first and only then encrypt things, but actually the opposite is better.
If you do Trust On First Use as many SSH users do, then you're correct that bad guys can interpose on that first connection if they happen to get lucky - but that's because they can authenticate as the "correct" server by presenting their own public key since you have no idea what the correct one looks like.