Documentation for vfs_aio_pthread and/or other relevant documentation may need to be updated to reflect that the aio_pthread vfs module only works on Linux and that pthread-based AIO path is built into smbd after 4.1.6. See discussion on samba mailing list here: https://lists.samba.org/archive/samba/2014-September/184958.html
Presumably fixed in 7ba243f3e72253992e6af3a2cc2bd0562170845f.