Consumer grade WiFi hardware being able to decode down to "femtowatts" of signal strength might be a bit exaggerated, but e.g. the USB WiFi adaptor in this video has a receiver sensitivity of -99dBm @1Mbps which is approx. 100fW. If we count the directional antenna as part of the receiver and add the antenna gain to that figure (this part of the calculation is somewhat disputable), we get the less-than-femtowatts mentioned in the video.
English subtitles for this videos are available, translations are always welcome!