That statement seems to be based on what one (apparently ill-informed) author stated. From TFA:
As one author on the KDP Community forums, Leslie Anne Perry, noted, “Previously, I have not enabled DRM on my e-books. My thinking was that I wanted folks to be able to download them to other devices within their own household. However, I think I will enable it on any future e-books. I’m not sure I want people to be able to download them as a PDFs [sic].”
For some reason, she apparently objects specifically to PDF.
I will note that other parts of TFA indicate this may actually be a rather ham-handed attempt on the part of Amazon to discourage authors from choosing to self-publish without DRM. That would not surprise me in the least.