No, you misquote me. I said that is is impossible to observe incrementality. It is rather trivial to estimate incrementality, statistically, but all statistical models produce estimates. Not observations.
How do you get the data if you're not a supplier? You contract for the supplier.
Not sure how you get this data if you are not the supplier.
You just mentioned in another comment that the key data needed to measure incrementality is impossible to get, from your perspective. [0]
[0]: https://news.ycombinator.com/item?id=33359741