B Julius Caesar Problem from a SpaceTime Physics book

[Nugatory]

Yes, that is more like the usual convention in SR problems: if you have two observers, make the spacetime origin the point where their worldlines cross. But this particular problem doesn't do that.

The origin of a coordinate system is the point that you’ve chosen to call $(x=0,t=0)$. The only reason elementary problems often show worldlines crossing there is to make it easy to say “they zero their clocks as they pass one another” without involving a clock synchronization procedure; this problem was designed to show how things work away from the origin.