GNOME and KDE are different desktops, yes. Each one also provides a toolkit that lets developers build applications that use the same look-and-feel as the desktop, and that share common libraries with the desktop. But they are certainly not incompatible. Many Linux users run GNOME and KDE apps at the same time.
To say that GNOME apps are incompatible with KDE, or vice versa, would be like saying that Java Swing apps are incompatible with OS X. An OS X user might prefer a native application to one written in Java, just as a GNOME user might prefer a GNOME-native app. But there’s certainly nothing to prevent a Java Swing app from running under OS X, and the Mac OS developers have attempted to make Java, Carbon, and Cocoa apps look and feel very much the same, just as Linux distributors have been doing with apps written with the GNOME and KDE toolkits.
And so I stand corrected.
It’s also beside the point I was trying to make. It’s good for everyone that apps written for KDE can run under GNOME and vice versa, but that still doesn’t make Linux an attractive market for commercial software companies like Quark and Adobe.