My iPad Pro came with an 18W charger. I suspect that probably also works fine. I think the only reason Apple don’t mention the 18W charger is that it’s no longer available and has been replaced by the 20W charger. If the 18W charger does in fact work then Apple should be more clear about that so those who already have an 18W charger don’t necessarily have to buy the new 20W one.
They WANT you to buy the new one. Magsafe Charger-gate
