I run PC, so do not have ProRes output capability, but here's how I see it.
If you export 8-bit video, there are 3 color channels, being red, green and blue. EACH of those 3 channels is 8-bit, which in effect provides a 24-bit image. If you add an alpha channel, that is another 8-bit channel, so 24-bit plus 8-bit = 32-bit. So I think whatever you may have read is just worded confusingly. A 32-bit image already includes the alpha channel.
Export a short sample with what you think are the correct settings, bring that clip back into Premiere timeline and see if there is alpha transparency - put the alpha clip on V2 with something behind it on V1 to see if V1 clip "shows through".
If you can perhaps post a screen shot of the Output Settings panel that actually shows the ProRes output options available, I or someone else here should be able to discern what is correct and offer some guidance, but I need something to look at to understand the Mac options that I don't have available on my PC here.