Update! Checked with a PSILY editor. Here's the response:

"You have the rights to your stories, though if you’re going to republish elsewhere, we do ask you credit us as the original publisher in the republished version with a sentence like “This originally appeared in ps I love you” with a link back to the original."

So, yes, you can repost to an outside-0f-medium publication with no problem - you own all rights -- but they ask -- but don't require -- you to credit them as the original publisher.

Which is consistent with what most venues want when it comes to reposting work.