Delivery is what happens if it’s not spam or abuse.

No inside information, but presumably this means Delivery to other organizations, which, among other things, includes maintaining outbound IP reputation, which is closely related to Spam and Abuse.

I was apparently not watching this well enough, sorry for the delayed response.

Deliver was because we ran the SMTP and queuing infrastructure at the time. We started as Gmail SRE, then split out some of the delivery and abuse services into its own team (SAD), then SAD got SRE, hence SAD-SRE =)