"Which explains why the Safari user agent string has so much extra information in it, e.g. KHTML, like Gecko — the names of other browser engines." Wasn't Safari/WebKit largely based on KHTML in the beginning? This doesn't say it wasn't, but seems to imply that there was a separation of the two that was more complete than it really was. I realize the OP is in a better position than I to know exactly what the reasonin…
KHTML had almost no market share - it only ran on KDE, and most KDE users used Firefox. Nothing recognized its user agent string, so reporting "KHTML" would have been as ineffectual as reporting "Safari" or "WebKit".