Let me put this to you this way. Previous versions of macOS 10.12, those which had specific kext in the OS, for Polaris10XT2, and AMD Vega traces, also has already traces of... Raven Ridge.
What is Raven Ridge? It is codename for AMD NPUs both mobile and desktop. Apple in 10.12.4 removed Kext altogether, from the OS, and left support for only Polaris 11. People who had been able to get Polaris 10 work in MacOS were left behind, with no support for the GPUs.
What does it indicate you ask? It depends on what are we going to get. You may be right that this year there will not be transition of iMac to AMD. Rather just simple spec bump. But next year is 20th anniversary of iMac, and Apple usually makes redesign of their hardware to celebrate it. That is how it happened with MacBook Pro, that is what is rumored to happen with iPhone, and IMO this is what will happen with iMac.
Apple is already working on something related with AMD CPUs, thats why there was already support for Raven Ridge NPUs. So they can extend it to other hardware as well.