It is, sadly: "Affects the amount of damage dealt by both physical and magic attacks, as well as the amount of HP restored by healing spells. Auto-attack potency is also affected"
If we break it down a bit on just the healing part:
"Affects the amount of HP restored by healing spells"
With just that, you would assume it increases healing done on you, rather than by you.

But if you take the first part:
"Affects the amount of damage dealt by both physical and magic attacks"
One would conclude it's damage dealt by you. Otherwise it won't make sense to stack this stat if it's dealt on you. With that in mind, you -could- conclude that the next part of the description would also imply that it's healing cast by you, rather than on you. That comma, though...