The linked DE is specifically a fork of a DE from 2002[0], so that would be why it looks like a DE from the 2000s. I imagine anyone wanting to use something like this has a fair bit of nostalgia for that style.
KDE Plasma[1] is the more modern-looking solution, but the aim of the DE is to do one thing well: provide a solid desktop environment. The stock styles look okay but can be customized in almost any way you like.