According to the pop-up tip, the Dequantizer can "clean up quantization noise, if the recording was created without dithering." This makes it sound like you wouldn't want to run it if your source audio is 16 bit and has been properly dithered.
But in looking at before and after spectrograms of a CD track, it actually looks like Dequantizer tries to "undo" a bit reduction somewhat. For example, foobar's "dither" actually is a dither plus noise shaping; you can usually see the noise on a spectrogram as a lot of "fuzz" around 20,000 hz.
Here's a spectrogram of an offset and gain adjusted track ripped from a CD upconverted to 32 bit float. It doesn't have a lot of high frequency and is mostly uniform noise above 8,000 hz, similar to what you see at around 20,000 for noise shaped bit reductions:
Here's the exact same track run through Stereo Tool's Dequantizer with default settings, except the two bottom quality sliders have been increased to their maximum:
Would it be accurate to say that the Dequantizer is a kind of "partial undo" (via reconstruction) for a bit depth reduction, regardless of dithering / noise shaping?
(By the way... the Dequantizer module is another module that is affected by "first run syndrome," meaning you will get a different, somewhat less intense result if it is the first time Stereo Tool is being run in a DAW session... Stereo Tool is at version 9.0 Beta 61 at the time of this post.)