In the community we already did the work for 64bit arm devices, this was before the work on Sailfish X had even started and was mainly pioneered by Ghosalmartin for the Nexus 5X (bullhead).
The userspace will still be 32bit and indeed that is a feature because there are no aarch64 compiled apps around for sailfish at all.
its the kernel HCI driver with the broadcom based BT chips which I already warned Chen against.