double optin cannot be achieved via API
True that it’s not directly possible via API but there is an indirect workaround by using a hosted form inside of Active Campaign. The ActiveCampaign form must have Opt-in Confirmation turned on, and the fields do not have to match your CF7 form fields.
In the pro version I created a field to enter the form’s id number. Then applied that to the $body array as $body[‘form’].
I have working custom code added to the pro version to create this work around.