Hecky,
In short, "So what?" It's a specious argument that's strictly on the highly technical side of things and is NOT of a particular concern to any ordinary user. PSD is a proprietary format that is based on the TIFF specification. So what.
The PNG file format used by Fireworks is identified as PNG and is readable by most other applications as PNG regardless of whether it has multiple layer or not ... it's irrelevant to them. Your source was from a 2009 thread. The author's experience may or may not have been accurate at the time. I know that I have never had a problem importing those files into Photoshop or CorelDraw/Photopaint... and I've been using Fireworks since around the mid-90s (when Fireworks was a MacroMedia product). At this time, many other up-to-date applications can and do read multi-layered Fireworks PNGs (and treat them as flattened ... and that includes ProShow). To the user, that's what's important. Graphic editors will simply flatten the imported FW PNG upon import.
So, it's NOT a "...furthest stretch of the imagination..." as you say, that PNG files support layers. However, as a practical matter, you only get layers in a PNG file if you're editing it in the Fireworks program. But, you also only typically get multi-layered PSD files in Photoshop too. However, it is a fact that any other application that edits a PNG file does so as a single layer to which multiple layers can be added ... and which becomes a flattened file when that application exports it to a PNG file. I'm sure that we will have to agree to disagree on this topic...
You must be a Photoshop snob!
These folks often think that the only useful app for creating graphics or editing images is Photoshop. Wrong! However, it is true that larger majority use Photoshop ... followed by any number of other graphical programs (because Photoshop was so gawdawful expensive) that do pretty much the same thing. Nothing wrong with being a Photoshop snob ... I've met many of them in my time of playing with graphics. But, too, many of these folks were simply surprised that there were so many perfectly capable graphical editing programs around that could do much of what Photoshop could do without as much of the steep learning curve or the exorbitant associated cost. So, you might be surprised that there are more Fireworks users out there than you suspect. Regardless, it doesn't matter a whit.
Dale