No, you are. It makes no sense to say an addon can't be stolen because it's public domain.
Taking someone's addon, stufftifying it and release it without permission of the original creator is stealing. You cannot justify this in any way.
Stealing: Take (another person's property) without permission or legal right and without intending to return it: "thieves stole her bicycle".
In no way does this count as stealing.
Anyway if modifying an add-on and releasing it as something else is stealing, then around 90% of all add-ons are somewhat stolen.