The most intensive operation I can think of that might happen at sshd startup would be host key generation, but at least in NixOS it looks like this is actually handled in an ExecStartPre= script, which I believe means it will happen before After= units execute.
Of course I sincerely doubt that distributions decided to add sd_notify support just for the hell of it, so I am sure there is a reason, it's just not overtly obvious, especially considering plenty of distros don't do this and sshd (and dependent units presumably) certainly seems to work absolutely fine.
The backdoor was designed to only be injected during the building of an RPM or Debian package. Arch never would have been impacted no matter what choices they made. They were trying to hit production systems while minimizing their potential exposure to other less important types of users beforehand.
I feel like a lot of people are really glossing over this point. What's happened here is that Red Hat and Debian have made a choice to patch OpenSSH in a way that opened it up to this attack.
It's a little ironic that e.g. Arch, which actually shipped the malicious code to end users since it publishes changes so much faster, as shipped never would have executed the payload (because they didn't patch OpenSSH).