This build is not efficient for dueling. It is not efficient for teamfights. It is not efficient for 1v5. This build is purely for splitpushing. If enemy team doesn't respond to you, you will take each tower in under 10 seconds. This build is for applying insane pressure that enemy team either has to match, or they forfeit their nexus.

With this build, you can take towers even when getting 3 manned due to Sion's insane HP scaling and shielding. Enemies must respond with multiple players or an extremely fed dueling tank-shredder, and while they respond to you, your team can make baron and drake calls without the strongest members of the enemy team to contest them, because they'll be busy dealing with you.

The reason you go mid is because Sion has a much better time against most mages and assassins than the bruiser fiesta in the toplane, and Sion also has access to raptor camps for easy CS while waiting for waves. Another benefit is a much easier time avoiding ganks due to Sion's shielding and E slow, as well as the ability to turn around fights with either Sion passive or a countergank, as mid is in easy reach for not just the enemy jungler, but your jungler too.

The most important reason for going mid, however, is the mid-lane tower. Opening mid-lane is incredibly important for having priority on objectives, and the short nature of the mid-lane means Sion can shove waves and take plates incredibly fast, and it also forces your enemy mid-laner to always match you instead of roaming, or else you will get a plate or two everytime. Even if important objectives are getting contested, you can get there immediately and turn the fight around with Sion's powerful R.

As soon as mid-lane is opened, you can start splitpushing and being a massive threat on the map so that either your team takes objectives or you take towers.

With mythic removal and huge item changes, Hullbreaker is in a weird spot to build around. Instead of doing a flat 20% extra tower damage, now Hullbreaker empowers your next attack against tower (and champions too) after stacking 4 basic attacks, which stack on anything, including minions, and resets if you don't basic attack anything for a while, similar to how Grasp of the Undying works. This empowered auto also scales off of your Attack Damage and Health. What this means is that Sion wants to stack AD and HP for a bigger empowered auto and also Attack Speed in order to reach 5 stacks faster.

Due to how many towers Sion can singlehandedly destroy, you will have a massive gold lead that means when you are full build faster than any enemy save for anyone fed, no one can stop you from running down tower after tower until you take their nexus.