While this is true, there isn't really a common browser distribution that uses that setup for non-OS X platforms, though. Chrome/Chromium/Opera use Blink now.
For all practical purposes, Safari is an OS X-only browser.
"Safari" aka WebKit + JavaScriptCore is also on iOS with a separate UI from OS X "Safari," similar to how you'd have a separate UI if you use WebKit + JavaScriptCore on Windows.
Whether it's popular on Windows is also irrelevant with regard to the fact that it does, indeed, work on Windows.
Speaking from practical experience, iOS' Safari is quite a different beast from the desktop version. `position:fixed` in a scrollable, element, for example, has completely different behaviors depending on the platform Safari is running on. You have to treat desktop Safari and iOS Safari as different browsers for development and QA purposes.
Windows Phone uses Trident + Chakra for its IE deployment, as well, so if we're going to make that argument, then IE is multiplatform, as well :P
It's available, but it's not used. I get what you're driving at here, but for practical purposes, Safari-OSX is a single-platform browser and Safari-iOS is another single-platform browser. You can't just target "Webkit" and call it a day.
For all practical purposes, Safari is an OS X-only browser.