-140 dBm is far beyond no coverage, yeah. -120 dBm is pretty much when LTE stops working (sometimes it can painfully stretch to -123 to -125 but usually not because of noise etc)
As near as I can tell, the smallest subcarrier 5G can use is 15kHz, the thermal noise floor for a 15kHz channel at room temp (300K) would be -132 dBm.
My guess is whatever chip doing the measurement simply couldn't measure that low accurately, or it reports "nothing detected" as -140 dBm.