Isn't k_nSteamNetworkingSend_UnreliableNoDelay Implemented?

k_nSteamNetworkingSend_UnreliableNoDelay is stated that the messages will be discarded when:

// - there is a sufficiently large number of messages queued up already such that the current message
//   will not be placed on the wire in the next ~200ms or so

https://github.com/ValveSoftware/GameNetworkingSockets/blob/master/include/steam/steamnetworkingtypes.h#L997C11-L997C42

But the in source code it seems it has no implementation:

https://github.com/ValveSoftware/GameNetworkingSockets/blob/master/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_snp.cpp#L335

So should we consider it is not implemented and do not use it? Or am I missing something? Thank you!!