New 2019 iMac Pro, why can't I install Mac OS Mojave on it instead of Catalina?

Hi all, I have a perplexing question that I can't seem to get a straight answer on (from Apple). I'm an Architect... I run a very specific BIM software (ArchiCAD r23) and since the release of Catalina I've been told by GraphiSoft (who makes the software) that they recommend waiting +3 months for them to work out any bugs in its compatibility with MacOs Catalina. The problem is that my 2015 iMac just doesn't seem to have the power anymore to run the software and the performance is killing my productivity. So, the answer is a new 2019 iMac Pro. I was told (from Apple in a chat secession) that when customizing it online I could select Mojave or Catalina OS and that is what it would ship with. I tried to place the order yesterday and got all the way to checkout and did not see the option. I called the 800 # with Apple and was told that was incorrect I cannot elect to have the older OS. I specifically asked about me downgrading the OS to Mojave when I received the iMac Pro and I was told that because of the hardware that was NOT possible. Factual? Not? I seem to see that people have downgraded from other OS's but I don't see any reference for a 2019 iMac Pro. I would love to hear is anyone has done this with the latest model.

Cheers!