These are all beautiful illustrations, but they don't feel iconic to me. Granted, there is a spectrum when it comes to detail, but isn't the purpose of an icon to distill an idea to its visual essence?
Glyphs and symbols are more likely what you're talking about. They are often referred to as icons in the tech field so I get the confusion with nomenclature.
Icons are mostly represented as visually interesting identifying artifacts about your product. That's why we call app icons _icons_.
The "icons" within the interface of that app, things that might tell you a menu is present or that a group of users is online, are symbols. Glyphs often conform to content. A good example of a glyph is an emoji or dingbats.
There's two sides of that. First there's trends, which has been flat-shaded minimalism for a few years now[0], but before that we had a range of photorealistic or cartoonish icon styles.
On the other hand, I do believe that most of these illustrations are way too detailed to properly function as icons. Styles and trends can change, but unless the author is working with a different meaning of the word "icon", they are always intended to be displayed at medium-small to tiny scales, staying instantly recognizable between a row of different icons.
They are beautiful illustrations absolutely, but I'd not use them as icons. I would use illustrations like this maybe for the front cover of a manual or brochure, perhaps a nice wall-painting(/print) at my office's reception, that kind of thing.
Now I haven't watched the videos (who here has?), so it's possible that some of these artists actually made less detailed and smaller versions of their designs, and it's just the largest one that's showing off in the display frame (it's actually not a very informative article on the whole, IMHO :) ).
BTW I've seen the term "skeuomorph" thrown around this thread a few times, but it's not a synonym for "photorealistic". Icons on themselves don't generally have skeuomorphs in normal apps or OS's, however they are more common in videogames. And when they do it usually pertains not to what they're a picture of, but to their function as icons, e.g. a paper tag/label, road/wall/door sign, icon (in religious sense), charm/bead, pill design, etc. In which case I'd argue the skeumorph is still more part of the UI than the icon design itself.
[0] counting "flat design" and "material design" icons as pretty much the same things here, when you compare them to the much more photo-realistic icons we had before.
I think detailed, illustrative icons can be iconic - human eyes are good parsing 3d things with complex shades, and the lighting and color can convey more information than just 2d shapes. I think the purpose of an icon is to be instantly and universally recognizable, as opposed to being a platonic ideal of the thing being conveyed.
It's just that skeuomorphism is out of fashion at the moment so this style look a bit dated.
Icons are mostly represented as visually interesting identifying artifacts about your product. That's why we call app icons _icons_.
The "icons" within the interface of that app, things that might tell you a menu is present or that a group of users is online, are symbols. Glyphs often conform to content. A good example of a glyph is an emoji or dingbats.