This is mostly a guess, but it accords with a few other sites. The requirement is most likely a "sanity" check to try to weed out cranks before they get into the system. I'd guess that philosophy, being a broad field, draws more than the normal share of cranks.
Try to find someone, such as an editor, that you can ask about the limitations. They may want some assurance that you aren't a crank, such as a recommendation from an academic.
But the limitation could also be on how often or how much you submit. And a review is natural for most publishers, though less so for preprint publishers.
Most journals are happy to post things from non-academics, so I assume the you won't be closed out entirely. But you may have to work your way in to the community slowly.
It is important that they do allow you to continue and don't immediately slam the door.
It does, however, seem that PhilPapers is primarily an index and bibliography of material published elsewhere, rather than directly submitted work. They describe themselves as a service to academics. While it also does preprints, you might think about submitting to one of the journals they index.