"This page assumes the status quo as of Plasma 5.7". Plasma is now on version 5.20 so the page you linked has some pretty old examples. The KDE folks are working pretty diligently to clean stuff like this up as they find it. Nate Graham's weekly blog documents this and other updates to the project.
Small half-joke: Only two words into the front page post I already hit a UX issue! (and a UI inconsistency)
> This week (KWin’s compositing code was almost totally rewritten...)
I then instinctively went on to the top of the article to check on the date, to get an idea of when was "this week" for the author... but there is no date. Oh, well :-)
Opening the post itself (instead of reading from the main page) does indeed change the UI and it now happens to show the date.
Loved the irony...
This all shows that even for people dedicated to it, getting UI/UX right is hard.
That page looks kinda out dated, and "contains some ideas how to improve the Plasma notification system". So a lot of that is mocks and not real screenshots.
KDE, to me, looks like someone designed it to be a touch interface.
Everything is so LARGE and padding between elements is YUGE. The tech is cool and everything seems to work fine, but something about the UI just grates me.
Also, KDE is the most customizable DE of them all, and it has the most powerful features of them all (by far). Given that, all these "but it looks kinda bad!" comments sound insane. It's like choosing a push mower over a motorized because you like the finish on the handle.
For instance, margins and paddings seem to be all over the place [1]. At least Gnome gets that part right.
[1] https://community.kde.org/Plasma/Notifications