You brought a fine point, We are still unsure how will the rage mode works, will it work as well on B450/x470 motherboards? We don't know that yet, there's a lot of blurry aspects with this new GPU from real performance, to drivers issue from last series and features, it didn't answer questions compared to what we knew, but people are getting even more hyped after a presentation.
From what I saw Rage Mode is nothing to do with CPU/Motherboard, one button Rage Mode is just extra overclock.
The CPU/Motherboard comes into action with the SAM mode ie faster access to memory.