Authentication failure despite config.ssh.keys_only=true

[bad]

Typical failure mode:

$ vagrant scp zorch default:
Warning: Permanently added '[127.0.0.1]:2222' (ECDSA) to the list of known hosts.
Received disconnect from 127.0.0.1 port 2222:2: Too many authentication failures
Disconnected from 127.0.0.1 port 2222
lost connection
$

There's #37 lingering for 2 years now and I only discovered that after coming up with basically the same fix and was about to create a pull request for it.

What is blocking progress on #37?