I'm sorry, I wasn't very clear.
Galio, pre-rework, got extra Ability Power by building Magic Resistance; this made him a hard-counter to Mages, especially burst mages like Leblanc, because he would get really Tanky and deal a lot of damage to them in return.
Riot is trying to return him to that/a similar state. Galio's damage reduction on his W scales with AP, as well as his Magic Damage shield I believe. By nudging him to build a little AP, which can be a little as a Rod of Ages or a Hextech GLP, they're trying to push him into a similar, anti-AP niche that he was in before.
Poppy, meanwhile, doesn't have that same niche. She isn't rewarded defensively for building offensive. Buying AD, for example, doesn't give her more Armor.
So while, yes, Galio will have the bonus of dealing damage in addition to being Tanky, he will have the main drawback that he won't be very durable VS a High AD enemy team. He'd be great when the enemy is Maokai top, Leblanc mid, and Kog'Maw ADC, but not so great when the enemy is Darius top, Zed mid, and Caitlyn ADC.
Meanwhile, while Poppy won't deal as much damage, she will be significantly more durable in a general scenario.