The reason is that "white paper" has a very specific connotation, and this paper is not consistent with the white paper.
Personal opinion: in science the word publication now means "to make accessible" rather "to publish in a conference" (the conference = publication thing is mostly CS) or "to publish in a journal" (like biology and a lot of non-arxiv physics). Pre-pub peer review isn't even required any more. arxiv (and scientists putting preprints on web sites) changed this. Of course, this is personal opinion, based mainly my observations and doesn't necessarily represent the mainstream opinion (yet).
I believe the parent referring to the common usage of "white paper" to refer to B2B marketing materials that are not often written in an academic style.
I was pleasantly surprised to find this was a self-published scientific article and not what I and dekhn usually think of when we hear white-paper.