eg. he says "the names of other browser engines", not "the name of the browser engine our code was originally based on".
The "like Gecko" part is a bit of a red herring here and mostly irrelevant, as that is simply what KHTML/Konquerer used in its own UA ("KHTML, like Gecko").
"That allowed me to tweak the string for maximum compatibility with the websites of that time. 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."
I interpret that as KHTML and Gecko are names of other browser engines that are listed in Safari's user agent string for compatibility purposes.