I believe that line in the documentation is there because the official Vagrant docs, as linked in the quoted line, mention that NFS compatibility with encrypted drives/directories is spotty:
Encrypted folders: If you have an encrypted disk, then NFS very often will refuse to export the filesystem. The error message given by NFS is often not clear. One error message seen is
<path> does not support NFS. There is no workaround for this other than sharing a directory which is not encrypted.
We’ve also has several reports of encrypted drives causing problems with Vagrant.
Thank you very much for sharing your solution, and I hope it’ll be helpful to people running into the same problem! However, so long as the Vagrant docs claim that Vagrant is largely incompatible with encrypted drives/directories, I’d feel uncomfortable if our documentation didn’t reflect that.