The 10900k is basically 2.5 times a 6700k at 5GHz, the amount of heat it produces is insane.
But at the same time I believe he tested 180º flipped 280mm AIO in pushout, as he's saying only 1 side has access to airflow and not like Form T1 or S1 (hinting he's talking about fans pulling air from inside of the case, aka pushout). And your results already showed the case is not sufficient to properly cool such high end cpus in this configuration: 80ºC at 2000rpm and the ryzen downclocking a bit at 4040MHz. Now consider his test limited at 1200rpm with a much hotter cpu running at 5GHz with fixed voltage...
But at the same time I believe he tested 180º flipped 280mm AIO in pushout, as he's saying only 1 side has access to airflow and not like Form T1 or S1 (hinting he's talking about fans pulling air from inside of the case, aka pushout). And your results already showed the case is not sufficient to properly cool such high end cpus in this configuration: 80ºC at 2000rpm and the ryzen downclocking a bit at 4040MHz. Now consider his test limited at 1200rpm with a much hotter cpu running at 5GHz with fixed voltage...