66MHz probably wasn't an 80486DX; as far as I can recall, those only went up to 50MHz. It would have to be a 486DX2, with the RAM running at only 33MHz. https://netlib.org/performance/html/dhrystone.data.col0.html says that a 486DX2/66 was about 31 to 51 Dhrystone MIPS.
https://www.riscvschool.com/2023/03/09/t-head-xuantie-c906-r... says the T-Head (Honey Badger) C906 RV64 core is about 2.3 Dhrystone MIPS ("DMIPS") per MHz, so we should expect the 480MHz C906 to be about 1100 DMIPS.
So the clock speed is only ≈8× higher, but the actual speed gap is probably closer to 32×.
And, for vectorizable workloads, we should expect the V extension to give us another 4–8× boost, so 128×–256× the DX2/66.
I have no idea how you reached this conclusion, given that the numbers you have provided are in total contradiction.
A ~400+ MHz processor today is definitely NOT desktop-class for 1993 when the average was a 486DX. I mean, I was there. You are a decade off, if you want to include all the architectural improvements that even a cheap CPU today has.
So it's definitely better than desktop-class for 1993.
Your overall point is valid, though I think you'll find that this particular calculation isn't correct. (The current year ends in a 6 so 30 years ago also ends in a 6.)
However, I just did a quick search for the Doom release date, which was 1993 and the original system requirements which is Memory (RAM): 4 MB and Processor (CPU): Intel 486DX (66 MHz).[1] So in terms of RAM this is 16x as much and in terms of MHz it's 6x faster on one of the cores. So it's definitely desktop-class for 1993. (33 years ago). As you state, it's much faster than desktop class in 1990.
[1] I only just looked at Google's AI summary, maybe it's wrong: https://imgur.com/a/LbhAmGp