Feels much less scary in 2024.. when was the last time you used HTTP for anything important?
(Edit: Even with HTTPS, there are other ways to MITM a connection because each trusted CA is considered an equal. For example, instead of doing a full-on MITM on a service which is likely to be detected[1], a better solution may be to obtain a certificate and target the specific user.)
(1) You need new host which hasn't been TOFU'd yet, and you need it to be exposed to internet with no VPN/mesh inbetween.
(2) You need password-based authentication _or_ agent forwarding, otherwise you won't be able to intercept traffic (with key-based auth, you can pretend to be the server, but that won't likely to last long). While I would not say that everyone uses key-based ssh, they should! It's an easy way to significantly increase your SSH security level.
As for HTTPs MITM, I don't see how this applies. They had to intercept traffic next to a server to be able to issue certificate - and I am pretty sure no one runs servers from the untrusted coffee shop connection! And you already have access to the server's datacenter, then there is no need to exploit public WiFi, you can intercept the server's traffic directly.
1. The ssh connection is intercepted, malicious server lets accepts arbitrary user's public key
2. Malicious server tries to connect to real server and fails - original pubkey authentication was channel-bound and cannot be reused
3. Malicious server now has to pretend to be the real server without knowing what the real server looks like. User immediately detects the shell prompt is different, the hostname is wrong, all the files are missing, etc... and disconnects.
(You can make some sort of contrived situation, like you only ssh'd to restart service, _and_ you used "sudo" with password _and_ you used ssh command so you didn't see the prompt _and_ this was the first command you did on your new computer... but this seems pretty unlikely)
And encrypted vs non-encrypted keys seems completely unrelated to connection interception attacks - who cares how the keys are stored on the computer? It's all the same on the wire.
The assumption was that it is the first time they connect. Which if so would mean that the key was configured out of band, but if we add that possibility you can do the TOFU out of band as well.
Yes, it is more common to transfer keys out of band - but it is getting a bit contrived.
...can use...
[1] http://notes.valdikss.org.ru/jabber.ru-mitm/
I have no doubt that there maybe a few dozen people in the entire world who would get attacked this way, but the chances it would be me are pretty much zero.