> SSH, on the other hand, has no equivalent of a Host header.

SSH cannot multiplex to different servers on the same host:port. But you can use multiple ports and forwarding.

You could give each machine a port number instead of a host name:

   ssh-proxy:10001
   ssh-proxy:10002

When you ssh to "ssh-proxy:10002" ("ssh -p 10002 ssh-proxy" wth your OpenSSH client that doesn't take host:port, sigh), it forwards that to wherever the 10002 machine currently is.

It would be interesting to know why they rejected the port number solution, but the only hit for "port" in the article is in the middle of the word "important" in the sentence:

But uniform, predictable domain name behavior is important to us, so we took the time to build this for exe.dev.

You can have uniform, predictable domain + port behavior. Then you don't need a smart proxy which routes connections based on identities like public keys. Just manipulation of standard port forwarding (e.g. iptables).