No, you can run most KDE apps on other systems, including Windows and Mac. I use Kate as my text editor on my windows work machine.
I used to be a KDE dev. We were largely volunteers, unlike a lot of other FOSS projects that had hired coders. The KDE e.V. funding largely went to server maintenance and helping students attend the annual conference (travel expenses! I benefitted from this a few times). Not sure if it’s still like that. In my era, KDE could easily get by on less.