12% damage reduction is a 13.6% increase in effective HP. 2,000 HP, with zero resistances after pen becomes 2,273 effective HP. That 273 damage is a lot less than 600, so this is a buff to glass cannon champions who buy PD.
A melee carry who has 3k HP and 30 resistances after pen gets their effective HP increased from 3900 to 4432. The difference of 532 is less than the 780 effective shield strength of the new Phantom Dancer, so it’s still a pretty sizeable buff.
However, there are some caveats here on both sides. The new Phantom Dancer can have its shield waited out. It doesn’t require an autoattack to proc. It also applies to incoming damage from ALL enemy champs, not just one.
So the new Phantom Dancer is now an anti-burst item rather than a pure stat stick. Its power is more visible and indeed much stronger in most modern scenarios, but that power comes at the cost of outplayability by only activating for short windows. It also has a low HP threshold, meaning the defensive function does nothing to prevent the player from getting low.