I could imagine that Canon, through the use of backside illumination, has kept the actual sensitive photosite area constant or near constant compared to the R7's, offsetting the effects of increasing the resolution to 40MP (which on its own would decreases the linear size by 15%). This would allow to maintain or improve the dynamic range through advances in sensor technology since the R7 (which, iirc, uses the sensor technology from the 90D, so from almost 7 years ago? Please correct me if I am wrong!), even despite stacking.
I doubt they would release a camera to the market that has worse low-light performance than the predecessor, especially since this new model is aimed to shift up-market a bit, also increasing buyers' expectations regarding image quality (and the target audience being wildlife photographers).