How does a volume mount differ technically from a bind mount at the kernel level?

Both ultimately use the Linux bind mount mechanism to make a host directory visible inside the container's mount namespace. The difference is entirely at the Docker management layer: for a named volume, Docker controls and owns the source directory (under its volumes directory, potentially via a plugin driver); for a bind mount, the source directory is whatever pre-existing host path you specify, with no Docker-side bookkeeping.