Hello David
First of all, in the official definition of NHRP as described in RFC 2332, the hub is not required to send a resolution request to the destination spoke. However, some vendors, like Cisco, choose to send an NHRP notification to the destination spoke to let it know that another spoke has been informed of its NBMA IP address and may initiate direct communication. Although not a required part of the NHRP resolution process, it may be used to optimize the setup of the reverse path, facilitating the establishment of a bidirectional tunnel between the two spokes.
Secondly, such a behavior only really has any value for Phase 3 setups. In Phase 1, spoke-to-spoke tunnels are never established so there is no need for the hub to inform the destination spoke that another spoke wants to communicate with it. Similarly in Phase 2, while the spokes do establish direct tunnels with each other, the NHRP resolution process is typically initiated by the spoke that wants to establish the tunnel. The hub replies to this with the necessary information to establish a direct tunnel to the destination spoke.
Indeed, the NHRP process that is described in the lesson is a generic one and does not represent the specific processes seen in each phase. However, later on, when Rene describes the various phases, he does state for Phase 3 that:
When a spoke router wants to reach a remote spoke, they will forward their traffic to the hub. When the hub receives the traffic, it will realize that another spoke is the destination and it will then send a NHRP redirect to both spokes.
…which confirms the behavior you see.
I hope this has been helpful!
Laz