From Adobe website:
.....
The next releases of Premiere Pro and Adobe Media Encoder will default to Apple Metal graphics rendering on macOS. This applies to new projects and existing projects. Apple Metal provides a modern and unified render pipeline for all users on that platform and will be the focus of our development on macOS going forward.
Metal isn't going to automagically get them balanced/distributed computation over Infinity Fabric. The extensions to Metal can be used to do that but it will take work. If Adobe is simply just trying to replace all of their CUDA and OpenCL with Metal it is far more likely their effort will probably be on rewrites of what they have rather than extending it to the new domain. If they hadn't already started on Metal substantially before they are way behind the curve on the transitions needed to leverage the Vega II Duo. ( folks who were doing multiple GPU with Metal would be ready to transition. )
Premiere Pro on Metal is needed simply to be viable across the whole mac line up. If they haven't sorted that out they they are unlikely to be doing work on a narrow vertical niche installed base of the Mac Pro. (numbers wise it will be too small a priority given the hole they are in. )