> The test was: "is there evidence for headphone output changing over time"

And the test was positive, given the test parameters. The author didn't want to extrapolate beyond that (I attribute that to the author's own biases against break-in: "I rarely hear break-in", "voodoo" when referring to break-in), which is their prerogative.

Not to mention that by their very criteria, "all I'm looking for here is a clear trend where the data changes smoothly from the start to the end reference", it did happen.

However, theirs is not the only test out there which shows differences over time. In my limited googling, the consensus seems to be "headphones do break in, even if the effects are limited".