AFAIK this is incorrect. There exist modulation schemes where the received signal power is evident. For Wifi the signal is usually > 10-100x the noise floor and it is modulated with phase-shift keying. So there is no technical problem with seeing a noise floor, then receiving a much larger signal with constant envelope and calculating an SNR.
ETA: Just to sharpen the point, this is clearest when considering digital noise and error correction. It is easier for error correction codes to indicate a corrupted message than it is to provide the correct message.
ETA: Just to sharpen the point, this is clearest when considering digital noise and error correction. It is easier for error correction codes to indicate a corrupted message than it is to provide the correct message.