Not a full answer but a partial from the other side of the fence: setting up my website and domain-name to send emails fully self-hosted - I finished setting up DKIM, DMARC and SFP on my brand-new, not-publicly-blacklisted IP address, and finally... Time to send some test emails out. Results:

[-] Outlook rejected my emails: vague reason given, but the fact is, Outlook rejects entire IP address allocations (for example, DigialOcean's entire VPS network), even individual DigitalOcean IPs which aren't publicly blacklisted. There is a proper process in-place for small upstarts like myself to get de-listed, but its not a given right that the process will favor the applicant.

[-] Gmail. Email received. No problem, no spam label. And I'm quite thankful to Google/Gmail for accepting my mail based on metrics other than IP addresses alone.

So while I can't answer why they're allowing obvious spam through their filter, I can say that at the present moment they are not as ruthless in blocking /16 networks (or whatever size) as some providers based on IP addresses alone (e.g., Outlook, E365).