Hello Xinran
The prerequisite that exists for Autonomous Systems within which iBGP peers are deployed, is that there must be routing established such that all iBGP routers within the AS can reach each other.
When you have a large AS with multiple routers, you would typically deploy a routing protocol like OSPF or EIGRP to ensure that this prerequisite is fulfilled. However, if you just have two routers such as those in AS12, they are directly connected, and they already know how to reach each other via their directly connected network of 192.168.12.0/24. So the prerequisite is fulfilled without the need to deploy a routing protocol.
In a real world scenario where you would typically have dozens of iBGP peers within an AS, a routing protocol like OSPF is absolutely necessary. But for this specific situation with two routers, it is unnecessary. Does that make sense?
I hope this has been helpful!
Laz