There's a section at the end about how we got to this approach. You're right that there isn't too much detail there, though.
In short, research on how people interact with information sources shows that we rely on the kinds of connections we're trying to preserve as important queues for remembering and mentally sorting information.
As other posters here have pointed out, purely looking at the informational content it'd be enough to copy over the current tab's history stack when opening a link in a new tab. That doesn't mean that this information is easily consumed, though: while the degree to which this is true varies from person to person, by and large our mind works in a way that we have a hard time grasping connections between nodes in a graph without seeing enough of it at the same time and being able to map it out. If the only way to get any insights into the graph is to look at each tab's history stack individually, that's fine for compuers, but just not too useful for people.
See Patryk's earlier post on this topic (and the post's bibliography) for much more background information: https://medium.freecodecamp.com/browserhistory-2abad38022b1