BG6HFD Bandwidth and ADC bit depth are not the same thing. Higher bit depth means higher vertical resolution, which allows for better detection of small signals. However, it also results in a larger amount of sampled data, which increases the processing requirements on the backend.
However, I agree with your previous point that DX10 is likely to use a 14-bit ADC, and probably uses two ADCs. A 18-bit ADC would be too expensive for DX10. It's also possible that the ADC in the FT710 is an upgraded version of the one used in DX10.