Its not the GPU in this case I believe the iMac Pro came out in 2017 and as a consequence doesn't have the Titan Ridge Thunderbolt Controller required to drive a single 6k display.Has nothing to do with the Intel CPU. It's all on the GPU.
Here's an article which explains it. Basically Titan Ridge (which was launched in 2018, a year after the iMac Pro) supports Displayport 1.4 and that's what is needed to run the 6k display.
It's basically the reason why the newer iMacs supports the XDR while the iMac Pro couldn't. The 2020 model certainly does and I think the 2019 one can too.