Show attribute widgets even if the Add to Cart form only represents one product
There are a few things you have to ensure you do right for this to work, as even in my little test environment to reproduce the error I screwed it up a couple different ways.


You must ensure the field you've added to your product type has checked the box to state that it should function as an attribute field on Add to Cart forms.
You must edit the field display settings using the form you've posted a screenshot of, but make sure you save the full form as the "Update" button on the expanded form doesn't actually save any changes to the database.
You must reproduce that for all relevant view modes.


With all of those things in mind, I was actually able to confirm the feature itself is working as advertised when properly configured: