I have no idea why I would "surely" mean that. Is "32-bit RGBA PNG" a synonym for "24-bit PNG with Alpha Transparency"? If so, I suppose I could have been more explicit, though I've never once seen such images referred to as that.
A 24-bit PNG would have no transparency, being 3 8-bit channels for RGB. Alpha transparency is a channel too!
To be fair, the PNG spec is really bad about referring to alpha as if it were something entirely different from a color sample. I have spent way too much time thinking about PNG minutiae for a project at work...
