The cp command does copy the file data but not the metadata. There is a reason we have come up with 2 words to distinguish them.
Rsync only cp the metadata when you specifically ask it to anyway. I haven't had a look at openrsync man page but I would assume it is the same in the case of the later.
Openrsync lacks the options of rsync for making exact copies.
Moreover, the OpenBSD file systems are unable to store all metadata that can accompany files in Linux filesystems or Windows filesystems, so that is the likely reason for removing the rsync options.
I also doubt that the developers of an utility for OpenBSD are also interested in taking care to preserve file metadata when copying to/from Windows, because the metadata access API is not portable, so a complete "rsync" utility must include specific code paths at least for Windows, for Linux and for FreeBSD. I do not know if the API of MacOS is also specific to it, or it is compatible with anything else.
Rsync only cp the metadata when you specifically ask it to anyway. I haven't had a look at openrsync man page but I would assume it is the same in the case of the later.