So, copying from his article:

"We are relying on publication editors to send in suggestions. They are our community curators."

He then goes and also says: "Does your story have to be in a publication to be Boosted? No. It does help a little, but publication editors have been suggesting stories from across all of Medium." but let's be realistic here, 99% of publication editors won't have the time or care much about other stories.

Then, if you watch Zulie's interview with some of the Medium staff involved in this, all you'll hear, is publications, publications, publications. https://youtube.com/watch?v=tKIaLk8N800&si=EnSIkaIECMiOmarE