Hello Walter
Yes you are correct, in the real world you only have control over your own network. As such, you have ultimate control over your outbound traffic, but you do not have ultimate control over your inbound traffic. However, as you can see from this example, it is possible to try to influence incoming traffic.
If you are admin of AS1, then you can cause the BGP routing of AS2 to prefer one path over another. Of course, the admins of AS2 are able to override such attempts by making the appropriate adjustments on their end. Remember, they have ultimate control over their outbound traffic too.
If you come up to such a scenario at your interface with another network, such as your ISP, it is always best to discuss what you would like to do before you implement it. Any attempt to influence their traffic may be considered hostile or inappropriate. Always discuss your intentions before applying any such BGP implementations.
More info about this can be found at this post:
I hope this has been helpful!
Laz