Does an NFS export with sec=krb5 *require* that the parent directory also be exported with sec=krb5?
You are using NFS version 4 (nfs4) which exports a single pseudo-filesystem rather than lots of separate filesystems.
This is specified on the NFS server in /etc/exports by fsid=0, and in your case is called /export (although it could be called anything). That is why you cannot remove that line or comment it out.
On the NFS client, this parent (in your case, /export) is seen as / (the root of the exported filesystem) which is why the automounter uses /home.