Seems like a basic statistical analysis of songs. Anyone who has composed or even played music will find no surprises here (unless you solely loved on atonal 20th century stuff).
However, the author clearly has knowledge of music theory, so I fully fail to appreciate the purpose of the article.
EDIT: Thinking about it, E minor is fairly common in phrygian scale (it's the tonic chord), however, phrygian, unlike dorian or myxolidian, which are used at least sometimes, is super rare -- its dominant chord, B dim, is dissonant, which kind of sucks.
Functional harmony kind of goes out the window once you stray from the major/minor modes anyway. You have to lean heavily on the root to maintain the tonality, or else the perceived key center tends to shift.
I think it's safe to say the effect of metal songs using phrygian is negligible.
(Also, songs are often written in one key, then transposed in the studio.)
I'm not sure why F would be more popular than D and A. One hypothesis would be that it has fewer accidentals; it's not a fun key for guitars, but many contemporary pop songs aren't guitar-driven anyway.
By that measure it's only natural that Em (iii) is more common than E (which is non-diatonic to C).
Emin vs Emaj - my gut would say Emin is going to fit in with keyboard/piano stuff easier, and there's more of that type music overall. When I think Emaj, I think guitar stuff (EAD stuff), and there's been less of that overall compared with keyboard-based music.
It’s useful for dreamy and somewhat static chord progressions though.
D/E/A - again, ime - were always "guitarish" songs. GCF always seemed to be more "pianoish". Just my own observation/experience - it may not be true of everything, but seems to be true about much of the stuff I've listened to (and tried to play).
A major (A/D/E) is probably the easiest key to play. G major is arguably the most useful, because you have access to all the important chords in open position: G, Am, C, D, Em.
Also, in this way we can say "C major" or "C minor" to refer to the key of the song. We use "natural" when its important to distinguish from a note that has a sharp or flat - e.g. C# versus C natural.
In Italian, how would you refer to the key of Db major - "Db natural"? How would you refer to the key of Db minor?