The referenced CS says "there are no technical differences between Products and Libraries..."
There is one HUGE difference: Products have on their Details page the "End Items" table. Libraries do not have this table.
The End Items table is a subset of the WTParts in that context - those with End Item = True. It only applies if you are using WTParts and Product Structure - and have designated End Items where appropriate.
The table, if Parts are maintained, becomes a very convenient top-level point for each Product Context to access all the actual items that you sell to customers from one place, and drill down to the details of each.