Standard PNGs don’t support editing. Simplifying it a bit, they are just what you said they are: losslessly compressed bitmaps (vs JPGs, which employ lossy compression, or GIFs which are also bitmaps, but only support up to a 256 color palette).
Fireworks PNGs contain a special header and extra data that allows them to retain vector and layer information. But they are not standard PNGs.
Fireworks PNG files contain a second “chunk” of data that other applications can’t read, which contains proprietary information about things like slicing, interactivity, and any Live Effects that may have been applied.
And finally, to address this:
What gives? Clearly there is way more to the .png format than just losslessly compressed bitmaps.
Yes, there is more to PNG than the standard. Extensions have been added to standard PNG that allow for animation, for example.
solved What really is a PNG? [closed]