AMCI compared a Stinger to its rivals in acceleration, cornering, and a closed-course lap to see which one will come out on top. Not sure if the test is fair when the BMW and Porsche are larger than the Stinger and a lot more expensive.
There is merit to your argument, but the Kia Stinger is a great car in its own right and was designed by former members of the BMW M team so there's a lot of German engineering in there, this a driver focused vehicle.
Its also important to keep in mind, that this is their first generation model to enter this segment. The Panamera has been out for 8 years already and much has changed in its lifespan. It's impressive none the less that the Stinger is being compared to the likes of the Panamera, as it really does offer similar performance at a much lower price bracket.