It's always been that Apple will say it works with the max RAM available at the time it first hits the market. If the Mac Pro hit the market and only 2GB sticks were available for sale at the time, well they could only say it could hold 16GB RAM.
This was before Intel Macs that the following was true. Well that I am 100% certain of... With PPC Macs, you can shove as much RAM in it as it can hold. Say a G4 says it can only hold 1GB PC2700 RAM (2x512MB) according to Apple because there was only a 512MB stick when the tower launched. Now that they make 1GB sticks, that PowerMac can actually fit 2GB RAM and it works perfectly fine. Apple just doesn't update its specs.
As for Intel Macs, I don't know. The Macbook Pros are weird. I've seen one where it said you could only install 3GB RAM (1-1GB stick and 1-2GB stick.) Someone said there was some kind of overlapping in terms of memory use so if someone stuck in 4GB, 1GB just wouldnt be used.
To be honest with you, I have no clue if filled it with 32GB of RAM if it work or not. I assume it would but I have no experience with the Mac Pro.
Bryan