who decides what is harmful? That's going to be like letting a Bible thumping Moral Majority zealot decide what's Art and what's pornography.
This is a law that will be used to censor.
More interesting question: if the Twitter app is made available through the Apple App store, then would famous Irish-based multinational company Apple be liable as well?
As a Christian, pornography is clearly sinful and harmful. But if you wish to use your free will to partake in it, then you do you. Non-consensual pornography (which is what this is about) causes harm to another person, which is why it cannot be tolerated.
This should be obvious IMO.