So the answer was indeed childishly simple.
The plugin Advanced Custom Fields is setup in such a way, that nested attributes are confusing because the field groups already look like nested attributes.
Instead, if you want to setup a nested attribute group, in ACF you go to --> new field group --> new field --> group
There you can setup your nested attributes. No addtional coding required.
(make sure you set the filter to appear on if post-type = product in the settings below.
Then, in your product feeds pro feed, empty your cache and you will see the grouped attributes show up in your field attributes (also, don't forget to populate the attributes for the products, or else they won't show up in PFP)