I'm not saying the stately isn't stupid to play against, but there are a lot of other problems when playing against him which is why the strategy looks unbeatable. Now I'm only gold 3 so take my words with a grain of salt, but the strategy is counterable and really REALLY weak if is team is behind.
The whole goal of initing Sion is create a massive amount of pressure. Oh there is a Sion smashing towers up top? Everybody recall and defend it. So all 5 recall and the enemy team takes dragon and a tower bot while you kill Sion. There's the massive problem on people go about taking this strategy on. They put themselves in a gold deficit trying to stop Sion.
Most of the time he only has health so either your toplaner, or a tank shredder adc should be more than enough to kill him.
Another point I made is the strategy is near useless if his team is behind and I stick to that. He can push as many towers as he wants but as long as demolish has a 45 second cooldown he is not out pushing 5 people down mid. You can easily win fights with good rotations while a top laner cough
if you want to really stomp sion cough stops the big man from getting turrets. The enemy team can't take objectives if they can't win fights. You can keep a massive gold lead if they can't get objectives. And you'll win a 4 v 5 as Sion desperately punches the inhib, flaming his team because they lost with a "broken" strat.
I'm not saying his damage to towers isn't stupid, but unless his team is super coordinated it will usualy end up as a dead strat,