Ah, I (finally) understand now. And furthermore -mgeneral-regs-only is architecture-specific; present for aarch64 but not arm64. Sorry for being dense.
So it would seem installing any *-dkms package by way of APT is going to fail on OSMC. Based on other forum posts (1, 2) I take it this is a known issue.
In any case, thank you again for accepting the wireguard install fix.