io_uring_recvmsg_out fields never used by liburing?

[TwoClocks]

Looking at the code in liburing.h, it looks like the fields in the struct are never used. Instead io_uring_recvmsg_payload requires the msghdr passed to the setup of revcmsg_multishot to get the lengths. The same lengths that look to be in the recvmsg_out struct (at least on my kernel).
What am I missing? Seems like it would be far less error prone to use the data in the struct, and make the API calls simpler.