I think it is also hard to be aware that you are in a trend when you are living it. Many of the things that pass as "clean design" at the moment are just a trend, and I promise you that they will look dated soon. This always happens. It happened with 80s hair, with 90s androgyny and it will happen again big text, extreme minimalist, big poetic photo backgrounds and so on.
The author is conflating two things which don't necessarily have any bearing on each other - equating fickle trends in design to mastery.
Flat icons, low-contrast fonts, parallax scrolls or whatever else are not necessarily an evolution in, much less mastery of web design.