There are two things I find amusing about the inevitable "it doesn't look as good as the same photo taken with a DSLR would have":
One is that, to the vast majority of people who look at that cover or other photos taken by the artist, they look like interesting, engaging photos. Most--and most by far I would guess--people don't look at photos like these and say "it looks flat" or "the micro-contrast is weak" or "it's not sharp enough" or "there isn't enough subject isolation" or whatever else. They see an interesting photo, and appreciate it.
And the other is the more salient point from an artistic perspective: Indeed it's true that if you took the exact same photo with an EOS-1D X mk II and an expensive lens it would be sharper, higher resolution, and have more dynamic range. It would almost certainly look better.
Except it's quite possible that the photographer couldn't take that exact same photo with an EOS-1D X mk II. As noted in the description, the subject might well have reacted differently to a photographer holding a huge camera with the shutter clicking away, or she never would have been in that particular location carrying a large camera, or whatever other nuances of the actual act of photographing another human being in situ might have come out differently.
It's the same if the photographer had tried to take the same shots with a large flash kit versus only natural light--the photo might be better lit, but what the person in it is doing would probably be less natural, and depending on the intent of the photo, less interesting.
If you've got a tripod, sufficient time, and are photographing a landscape or model, sure, a large-sensor, large-lens camera is going to produce a better photo than an iPhone in every way. And sure, there are photos that an iPhone is incapable of taking for technical reasons--available light, field of view, depth of field, bokeh. But following around a human trying to take a certain kind of photo, the act of photography itself changes the result, possibly for the worse.
Heck, I'm just a casual photographer and I've noticed it plenty of times just taking street photos while on vacation. I hold up a smartphone to take a picture, nobody even notices. I hold up my bigger camera, people look at me, stiffen up, react like they're being photographed. Occasionally the effect can be positive, but more often it results in odd expressions and less natural behavior.