I don't have info, but this is the case on other devices like the Pixel XL. it's mentioned on a github issue here:
https://www.bountysource.com/issues/61605333-device-port-marlin-google-pixel-xl-1st-gen

It seems like they figured a work around?

The notes say:

Specific features
Dual-boot
This is an A/B device, so dual boot is possible: I'm currently running Android P in slot A along with Halium being installed in slot B.
TODO: Add dual-boot installation instructions
No recovery
A/B partition scheme means there's no recovery partition anymore. Instead, it's located in initrams of the kernel. At normal boot, bootloader puts skip_initramfs flag into cmdline and Android obeys that. Halium needs initramfs, as some scripts are run there, so there's a dirty hack in the kernel to ignore skip_initrams. Thus, installing TWRP will break Halium and prevent it from booting.
TODO: Research the way Android boots and replicate that behaviour in Halium

I think the answer might be:
Halium needs initramfs, as some scripts are run there, so there's a dirty hack in the kernel to ignore skip_initrams.