I’m in favor of browsers going with anything that isn’t Chromium, to slow Google’s complete ownership of the browser.

Guessing: it's not Blink, but it's easier to work with than Firefox's engine. I know people fork FF, but I vaguely recall hearing the engine itself was harder to separate and integrate into other systems. I'm sure someone closer to the issues can correct me.

It also runs on iOS/iPadOS and Webkit is the only allowed engine on these platforms (outside Japan).

It was a good choice for macOS and also works on other platforms.

Battery life and performance on MacOS

chromium is the devil and I understand them not using it, and when Orion started development firefoxes engine wasn't really re-usable iirc. Plus it started life on MacOS