Jay TBH, I don't think that's the case. HBSD is costantly kept in sync with FreeBSD and once in a while some HBSD changes also makes it back to FBSD. HardenedBSD is 'just' FreeBSD + all the PaX exploit mitigations which FreeBSD never wanted to merge.
I remember Shawn saying they were considering the option of removing the Linux ABI, but years later it would appear they eventually decided to keep it. Looking at the sources, it seems the compat layer is maintained and up to date and that they also merged the new linuxulator support for arm64.
However, as the Linux ABI is definitely not among the priorities or the goals of the project, I wonder how seriously it's been tested and how well does it plays with stuff like MPROTECT