I do not understand why Lineage insists on waiting for eBPF back ports when PostmarketOS has a far newer kernel running on the same hardware.

Core Android functionality relies on eBPF in a way that PostmarketOS does not. PostmarketOS is much more of a linux distro than Android is. They are not very comparable.

AOSP patched kernels still include some features that are not in the mainline version. The LineageOS folks are working on support for mainline kernels, but AIUI it's not there yet.