Zen is a Firefox (skin? fork?), and the Firefox community is their own niche (and IMHO, more Linux/Windows-focused rather than Mac). I think Kagi's focus on Mac made WebKit a natural choice, even though Firefox would've been the more platform-independent choice (as in being able to make the one browser for all three platforms, rather than bringing one to Mac, then another to Linux and to Windows).
Firefox has never really felt at home to me on Mac. I've used Safari (and previously Orion) ever since letting go of Chrome for most personal use.
I’d rather see Orion continue to be developed alongside Zen though, because goodness knows we need more non-Chromium browsers.
I don’t think Mozilla is going to be able to escape this so long as the they continue to use their current UI stack.
I don't think most people care about the engine that powers their browser. They care if it's fast, renders the sites they need, has a pleasant user interface, and doesn't crash.
A Gecko-based browser can do all of that.
FWIW, for me personally, particularly with newer versions, there has never been a moment when I felt Zen didn't fit on my Mac just fine.