I don’t know exactly what Zoom and others are doing precisely, but typically this means you have two ADCs, set at different reference levels, capturing the same signal. You can think of it as a low level ADC and a high level ADC. When you combine those two signals with DSP, you can get a sort of floating point representation. It is very tricky to do this in a way that doesn’t have distortion when you crossover between the two ranges, but for something like Zoom, the utility is probably more the focus.
I have had a hard time explaining to people over the years the value of floating point in DSP as the SNR is actually floating with the signal level. Not from the converter’s perspective, but in the actual math. The most obvious value is that gain changes don’t truncate precision. RBJ is obviously right in that if you have more precision in the registers for fixed point, that is also true. But in the case of floating point, you could gain a signal down by 100dB then gain it back up by 100dB and it is lossless. That can be handy in DSP. However, the second you mix that signal with a signal of differing level, all bets are off. I’m a fan of floating point for DSP. It’s not perfect, but it is convenient and has some advantages over fixed. Just my opinion.