WebKit was originally based on KHTML, but Apple still founded WebKit and controls its source code. Even if WebKit was just Apple's name for their internal fork of KHTML, I don't see any reason they'd retire it.
Blink was originally spun-out of WebKit too. Similarly, Google controls Blink, and the two have diverged significantly in the intervening years. I suspect patches for any of them won't apply cleanly to the other two.
Regardless of their origins; KHTML, WebKit, and Blink are now independent pieces of software.
It's still unclear to me why anyone should expect Apple to retire WebKit for iOS.