Are Nature Letters still considered primary articles?
I was not previously familiar with the distinction between "primary articles" and "secondary articles", but the distinction is explored in this very helpful link.  After comparing the paper with this document, it seems clear (even) to me that it is a "primary article": they describe results of a rather massive experiment which has not previously been published, and they contribute their own statistical analysis. I didn't see any evidence of a literature review or anything like that.

Also see how Nature describes its own Letters here: they are short reports of original research.

Forgive my naivete, but: why does it matter?