Are there a lot of apps where the keyboard shortcuts on iPad differ from the ones on iPhone? They should function the same on either device in Drafts, and from a developer perspective I would think it would be harder to make them not work on iPhone than to just let them be the same.
IPhone does not have the benefit of the iPad overlay to help discovery. In Drafts you can at least enable shortcuts for actions to display in the list (in … action list options)