This is an interesting thread discussing floating point with SNR. I've had many of these discussions before and it is tricky to navigate, somewhat because of terminology. You have to consider separating the concept of traditional converter quantization noise from what is going on numerically. What these companies are attempting to do is to construct a numerically higher SNR signal from two quantized signals. To Scott's point, that is extremely difficult to do without adding more noise or distortion, but ignore that for a moment.
Back to numerics... A floating point number maintains its full 24 bit mantissa precision at any value within the exponent range. In a digitally created signal (no real life recorded noise), you can create the signal with a peak level of 0.001, or a peak level of 1.000, or a peak level of 1000.0 and they have exactly the same SNR, -144dB. The way we measure SNR, using a steady state sine tone of 0dBFS is not going to show any advantage for floating point. It should measure identically to 24bit fixed. In truth, the "quantization noise floor" is actually modulating with the signal values, with the highest quantization noise at the peaks of the sine tone and the quantization noise reducing as the values approach 0. However, this isn't really an advantage with the way we hear or with the way we measure. The advantage is that real signals have quiet sections and louder sections, and you can compress the signal, or turn up the quieter sections, without turning up the quantization noise there. So, the goal of these field recorders is to essentially use two different preamp gains to capture quieter signals with higher gain and louder signals with lower gain, and then try to combine them into one signal. Since the gains are different, you need to go to higher precision numbers to reconstruct the combined range. Calling this floating point is somewhat misleading as I doubt the difference in preamp gains is more than 20-30dB. At best, it is similar to 30 bit fixed point audio, maybe as high as 180dB numerical representation.
But back to that difficult to do piece... Let's say you have a quiet section of the music, you really just want to use the quiet converter as adding in the loud converter will just add noise into your signal with not much value. Then the music gets loud and you want to use that converter, the switch to that converter is very difficult to do without adding noise or distortion during the switch. You get the idea... So the idea that you would switch converters during the middle of every single cycle of a sine tone during an SNR measurement seems like a worst case scenario. Ultimately, you would be better off just sticking with the loud converter as that will dictate your quantization noise based SNR.
I think it's more useful to think of techniques like this as an attempt to save a poorly recorded, or challenging to record, signal than a way to increase SNR. It's just a way of recording the signal with two different preamp settings and merging them conveniently into a single file.