I think it depends where the weight is. For example. 10kg on your body would have more effect on time than 10 kg placed optimally on you kart.
In the video the added the weight quite low. And its not moving. A heavy driver is moving alot on an real kart which affects the behavior more.
As i rememember in the wtcc years ago as the added up to 30 kg for the leaders the spoke about 7 kmh less on topspeed and we are talkung about 1200 kg cars. So i think 10kg in a racing kart with about 30 hp would make around 3-5 tenth depending on track.
In my dd2 around 4 kg of gasoline are about 2 tenth on a lap. Maybe its just my wrong experience but when i filled up i was always around 2 tenth slower