That's correct, as far as I know. I'm not aware of any support in the planner tools (like
Keyword Planner) for product partition criteria, but you might ask on the
AdWords Community to see if one of the (non-API) product experts there has alternative ideas. Note that when posting to a non-API group, you'll want to use the term
Product Groups instead of product partition, since that's the name for partitions in the user interface.