I see the photo of the sensor atop some springy bed slats, but don't see mention of how the sensor setup distinguishes the weight of of mattress above springy bed slats from when a person is atop.
water is 1kg/dm3, and acording to this website (https://www.aqua-calc.com/page/density-table/substance/human...) human is arround 1000kg/m3 which make sense.
So, yes matress is light compared to human, I guess it does nothing to the sensor pad.