Why doesn't validation catch when the two sides of a reliable connection disagree about whether the session is ordered?

It doesn't actually matter to the sender whether the session is supposed to be ordered, and so it doesn't actually matter whether the two sides agree. The sender has a direct and perfectly reliable view of the messages in the sequence as they're generated. In essence, the order of the messages is whatever order the sender chose to send them in. On the other hand, the receiver has an unreliable view of the message sequence. The purpose of a reliable messaging protocol is to reconstruct a reliable view from that unreliable view. Therefore, the receiver actually has a choice about whether the original sender order is important and should be reconstructed.

This is different than saying that ordering is useless on the client. Client and server are ideas about the organization of a system; sender and receiver are particular roles in a protocol. A client could be both sending and receiving messages.

Next time: Overriding Protection for IPSec