It's less a nerf and more of a power shift.
With Sion's Roar of the Slayer, you get some really long-ranged poke. If you max it first, your poke is extremely strong, even when building full tank, and your poke is super safe.
This wouldn't normally be a problem, except for the fact that it costs so little. 35 mana is way too little for how strong a poking tool it is, especially considering how deceiving its range can be when used through minions.
These factors give Sion an inappropriately strong early game for how strong his late game is, so the mana adjustment is there to keep his laning phase more in check, and expand on the weaknesses of bullying Sion early to delay his lategame power spike.
He's also getting the mana cost reduced on his Decimating Smash at later levels, which is a pretty good compensation buff. This improves his early game if he maxes it first without making his laning phase too oppressive to go against.
Also, don't forget the indirect buffs he's getting. The new Cinderhulk enchantment should be absolutely insane on jungle Sion.