This was one my favourites parts in the article. An the real piece of the pie comes right after it: there's a screenshot with like thirty icons and they're different enough that you can tell one from the other.
Ever since they made this move to every icon being a minimal shape with the 4 Google colors, I haven't been able to quickly differentiate between app icons. I have to read the labels half the time now cause they're so non-indicative of the app's purpose.
This sounds like a great title for a really good book.
Funnily enough, The Lost Art Of UI Design sounds like a potentially very mediocre book to me. One of those stretched-out "self-teaching" design guides that listicles will recommend you. It chooses a couple of very specific rules of thumb, it gives way too much importance to them, and somehow manages to water them down to 250 pages. Probably written either by someone who's a skilled writer but barely knows enough about the industry, or an experienced specialist in the field who unfortunately can't write for life.
Hell, I don't think they're original to Flash to begin with, more like borrowed from Photoshop when Adobe bought Macromedia (this is purely my speculation; but Photoshop is much older).
"oooh let's not put too many buttons to scare users away", "they got 22 inch screens, let's just waste space for no good reason, it looks modern and airy"
The crucial difference between then and now is the amount of whitespace.
Usage can't be it, because I use a ton of software that I despise and get depressed by, but only because I get paid to do so, or the app is so niche there isn't a viable replacement.
You'd have to actually talk to your users. But that requires genuine human investment, so it doesn't scale. It doesn't come in the form of a third-party SaaS with a pretty dashboard, generous free plan, and integration via single <script> tag.
In short: they could measure this, they just don't bother to.
I seriously think that's what's going on.