Julian,
The receiver performance numbers apply in a noise-free environment. Ultimately, what matters for the receiver is signal to noise ration (SNR) rather than the absolute signal level.
In a 20MHz channel, a sensitivity of -90dBm translates to a need for about 11dB SNR. (thermal noise floor is around -101dBm).
In your example, if we pick up -90dBm as CCI noise, we would need a signal level of -79dBm or better to decode the data.
A less sensitive woudl still be affected by the noise, and need even more signal power to decode the data.
Hope that helps.