There are required steps to follow but none are "have x users" or "see a lot of spam". It's mostly "follow proper DNS steps and guidelines in the given format" with a little "show you're doing this for the intended reason rather than to circumvent something the PSL is not meant for/for something the public can't get to anyways" (e.g. tricking rate limits, internal only or single user personal sites) added on top.
"Projects that are smaller in scale or are temporary or seasonal in nature will likely be declined. Examples of this might be private-use, sandbox, test, lab, beta, or other exploratory nature changes or requests. It should be expected that despite whatever site or service referred a requestor to seek addition of their domain(s) to the list, projects not serving more then thousands of users are quite likely to be declined."
Maybe the rules have changed, or maybe you were lucky? :)
Thanks for the note!