Well, I used it every day. Apparently it was enough of a thing for Apple to notice it, too (or let's face it, the Mozilla people - like David Hyatt - they hired to gain the knowledge).
Gecko wasn't GPL either. And Gecko was actually originally meant to be embedded by others, before the community decided to productize and do Firefox (originally Phoenix).
I'd like to think they noticed it because it was good. KHTML at the time had quite strong CSS support compared to its competition (not necessarily the best, but it competed well), had done some important advances in l18n support compared to others (e.g. strong support for bidi text layout), and the design of its render tree which e.g. allowed for hardware-accelerated layer compositing was later adopted by competitors like Gecko. It was a pretty good codebase.
Sure, it's been a decade since those events, and the majority of the engineering effort by now lies chiefly on the WebKit side of things. Apple employees working on WebKit have done a ton of good work, as have other stakeholders. Apple was an important factor both in terms of workforce increase and popularity increase for the project. But I think it's very legitimate to continue to credit the KDE community with its inception.
