It is a closed fork of FirefoxOS, which worked with Android drivers. They didn't employ libhybris, but the trick was to compile Gecko for bionic instead.

Honestly, having KaiOS version of Nokia N9 would seem as disgrace of original N9 to me. If current KaiOS phones are anything to look at, it might have even lower specs than original (Nokia 8810 4G has 512 MB RAM), so coupled with closed bootloader it probably wouldn't make a nice device for porting. And as for KaiOS itself... they started based on open source project and made it completely closed source except the parts they had to open by license (Gecko engine), then turned to Google.