FT8 isn't actually below the noise floor if you look at the bandwith the signal is detected in, rather than the 2500 Hz reference bandwidth.

https://tapr.org/pdf/DCC2018-KC5RUO-TheReal-FT8-JT65-JT9=SNR...

Has details, but basically you should add 26 dB to account for the difference between 2500 Hz and the 6.25 Hz bandwidth each FT8 tone is detected in.