Hello Jianxun
The reason why multihop is being used is because it is the loopback addresses that are the update source. This means that the loopbacks are actually two hops away from each other, even though the routers are directly connected. If the interface IP addresses were used as the update source, then no multihop command would be necessary.
You can find out more info about this at the following lesson:
In the link above, take a look at the second scenario which is similar to the situation in this lesson.
I hope this has been helpful!
Laz