That claim isn't even remotely plausible unless one of the following conditions is true:
Phase has managed to create a sensor, read circuitry, and A/D converter that keeps noise +/- 1 ADU all the way to the black point, which must be exactly zero, and you accept 4 levels as an acceptable number of tonal gradations for the 15th stop.
OR...
Phase has invented a sensor technology that does not have a linear relationship between photons striking a pixel and the ADU readout for that pixel.