How does this make it harder to move out of AWS?

If anything, their use of NFSv4 means there are plenty of competitive offerings if you decide that performance, security, or physical access constraints dictate migrating off their service.

If you don't want to manage your own Linux/BSD/etc. NFS infrastructure, Oracle, Netapp, and EMC will all happily sell you a storage appliance that supports it. I don't see much lock-in here.