The author looks like they've only looked at the color of the dead space so probably not significant for this specifically.

The trend against skueuomorphism maybe equally relevant: that early example is a descendant of Apple's previous brushed-metal UI. Though even among the flat ones there's been a trend toward lightening.

It'd also be interesting to see what area the author picked on each screenshot: a big difference, at least before Tahoe, if you decide that the Finder sidebar or top bar is what you're going to look at.