Not entirely sure if the article is aimed at a web audience, but if it is, this would be an awkward justification because the unit "pixel" in CSS is normalized w.r.t DPI.
In the first case the amount of mm would be different for each display. In the second case the amount of pixels would be different for each display.
Perhaps this quote from the article provides a hint:
> Ok, I get the even number thing, but seriously, why not 6, or 10? The majority of popular screen sizes are divisible by 8 which makes for an easy fit.
From this I'd say it is about pixels. But why call it "pt" then? And yes, how does this give useful results on high resolution displays?
I think the article is really confusing.
The point is that the author says that the screen size in pixels should be an integer multiple of 8pt, which doesn't make sense.