As per the below statement :

The iOS 'Whiteboard' feature differs in behavior from the whiteboard functionality on desktop clients. Therefore, whiteboard related settings in the Desktop client will not apply to iOS whiteboard file users.​​The app uses share image file technology to start the whiteboard share and stores in the device Photos.

The feature that is to be added here is to enable annotation capability when a whiteboard is initiated by a desktop client the IOS user on an ipad or iphone should also have the annotation feature.