It just means that the original VMDK disk image (inside the OVA) is read-only. AFAIK that is the default for how disk images within OVAs should always be. When a new VM is created from an OVA, a new (read/write) VMDK disk image is created from the read only one within the OVA.

This is in contrast to our VMDK build, which includes a read/write VMDK file which you can attach straight to a VM and use as is. If you wish to create more vanilla VMs from a VMDK build, you need a clean VMDK disk image (i.e. extract again from the original VMDK zip file).

I hope that makes more sense?