Come on....
That's just painfully wrong. They attackers turned on SSH on a specific port, created an additional login and turned off security functions. Those are all VERY detectable.
The ACTUAL information from GreyNoise says
You could say "the attackers took steps to try and avoid detection".
And then this:
It might be more honest to say "The most recent ASUS firmware does not remove the attackers access to systems"
An ASUS firmware patch (in addition to fixing the original 'undocumented authentication bypass technique') can simply re-disable SSH, nuke any SSH keys, and then turn off/on anything that was part of the deployment package in this attack. Patch notes "if you had previously enabled ssh, added your own SSH keys, enabled remote access, disabled the security features on your router, you'll need to do that again."