How does the CharacterMovementComponent keep its physics deterministic and its deltaTime fixed?

It might depend on what we mean by “send” here:
I agree it is scheduled immediately, and then sent when the next actual UDP packet is made and sent.

AttachmentSendQueue.Enqueue(OwnerIndex, SubObjectIndex, reinterpret_cast<const TRefCountPtr<FNetObjectAttachment>&>(RPC));

And then the data that come back, for correction, is subject to a send rate limit:

»   if( (ServerData->LastUpdateTime != GetWorld()->TimeSeconds))                                                                                                      
»   {                                                                                                                                                                 
»   »   const AGameNetworkManager* GameNetworkManager = (const AGameNetworkManager*)(AGameNetworkManager::StaticClass()->GetDefaultObject());                         
»   »   if (GameNetworkManager->WithinUpdateDelayBounds(PC, ServerData->LastUpdateTime))                                                                              
»   »   {                                                                                                                                                             
»   »   »   return;                                                                                                                                                   
»   »   }                                                                                                                                                             
»   }                                                                                                                                                                 

This ends up checking two configurations: a maximum send interval, and a value derived from client net rate.

Anyway: to double down on the answer to the original question up top: CharacterMovementComponent does not keep its physics deterministic, and does not keep a fixed deltaTime, and corrections are only sent and acted on when the movement position delta is “too large.”