[fjtorres]

They're only claiming correlation, not actual causation.

DRM-free titles earning twice as much as the DRM'ed ones could simply be an indication that business-savvy publishers, the ones who know how to promote their titles effectively, also know that DRM does nothing meaningful for them. It does, however, allow for TTS, simultaneous reading on unlimited multiple devices, and easy conversion to non-Kindle devices. All of which add marginal value to consumers.

So it says nothing about the quality of the book itself but it says a bit about the publisher.
In any industry, there is a body of knowledge of "best business practices" that correlate with success even when they don't always cause it or guarantee it. DRM-free might be in that category, not a guarantee of quality or success but it clearly doesn't hurt.