It seems more likely that Apple would've adapted this from the proven technology that they currently use for Apple Books and everything else, TextKit (which first appeared in OpenStep). https://developer.apple.com/videos/play/wwdc2021/10061/
Can't speak to Apple Books, but at least Pages.app (and iWork in general) use a separate text engine from TextKit, focused on higher fidelity at the cost of performance -- optical kerning, etc. (Terminal.app also does not use TextKit.)
I was under the impression that when we got new css in Safari, in the next software cycle those same features ended up in Books. It wouldn't make sense to give it a different rendering engine... but then I've never been able to find much in the way of which epub readers used which rendering engines anywhere.
