I'm not really sure this is a bug since it sort of behaves as documented, but at the moment BasicForm settings have to be passed using the initialConfig parameter, which is set in the constructor function of AbstractComponent.

Although not listed as configuration options of FormPanel, the FormPanel class accepts all of the config options supported by the Ext.form.Basic class, and will pass them along to the internal BasicForm when it is created.

Note**: If subclassing FormPanel, any configuration options for the BasicForm must be applied to the initialConfig property of the FormPanel. Applying BasicForm configuration settings to this will not* affect the BasicForm's configuration.
So it's not possible to pass the api and other settings when using the Ext.define class definition structure that is commonly used in the MCV examples. I think it would make sense to be able to this without having to do stuff in initComponent (this.getForm().api = this.api