Advancing the main story line is decoupled from the rest of the exploration of the open world and other content. In fact, the main story line is pretty optional provided you reach a certain point of the story:
. Is the main story line linear? Sure, because it has specific story with a specific structure to tell, but it doesn't get in the way of its open world.
In fact, Horizon does something that I don't see a lot of people praising it for, and it's kind of understandable because it's very well done and so subtle as to be almost unnoticeable. The dialog in Horizon, both for the main story line and side quests and such, is extremely dynamic. It can change completely depending on what you've done, who you've met, where you've been and what you've said in the past, and it does an amazing job of keeping multiple parallel narrative threads (from main story line to sidequests) not only coherent but also bouncing references between each other. That does a lot to prevent the usual disjointedness that a lot of open world games with a narrative emphasis have some times.
