Yeah, dual-rank b-die would be much closer.
It's an odd setup.
The plus is running IF at 2000 1:1
I think TPU's bench is normally run at 1800 1:1, so the loose timings were likely necessary to get 4000 with IF 2000 1:1 (this is rare on Zen 3).
I would bet that in some cases the 4000 C20 wins with the higher IF speed 2000 vs 1800 1:1. Probably more of the eye-candy games like Cyberpunk and Assassin's Creed will prefer 4000, while what I call 'twitch' e-sports titles will prefer the lower latency.
I personally would have rather seen standard bench setup though, with more CPUs tested, even if the tested set of games were smaller.
The standard test bench at TPU IMO represents what most DIY enthusiasts will build (DDR5-6000 C30 / C36, DDR4-3600 C14 or C16). Every other site seems to have some implausible config that makes their data unrepresentative.
This setup, unfortunately, kinda fits in that 2nd category.
Should I test 5800X3D or faster RAM speed?
I'd prefer you used the standard bench setup, regardless of CPUs tested. I don't have a basis for comparison using DDR4-4000 C20.