It does not tell the difference, but you shouldn't use PNG for "3dpd" anyway. PNG is for drawings, charts, screencaps etc. For everything else you should use JPEG.
That being said, the results are surprisingly good even for photos. It is worth noting that the thumbnail images generated by Dietchan are actually 4x larger than they are displayed on the page (this is to support HiDPI screens and to give you more flexibility with the CSS). When the dithered image is downscaled for display, the result is almost indistinguishable from a non-quantized thumbnail.
Oh, and in case it wasn't obvious, only the thumbnail is treated with pngquant, not the full-size image.