Well, that is not true. SNR does not depend on whether the system is implemented as discrete or continuous. All the things affecting the SNR being the same (like OSR, quantizer level, loop order, etc), SNR of a well designed DT DSM and CT DSM will be equal. In fact a CT DSM is derived from a DT DSM using impulse invariance. In other words CT DSM realizes the same Noise Transfer Function ( NTF) as DT DSM. So there is no reason why one would perform worse/better than the other in terms of SNR.