If you care about benchmarks, a Pi4 will score around 200 points (1 core) or 550 points (4 core) at 1.5GHz on GB5. At 2GHz you can reach around 700 points multi-core.
Ryzen 2700 will easily go above 6000 points in multi-core bench without overclocking.
So even in a theoretical, embarrasingly parallel workload with minimal sharing between cluster nodes, the Zen will be faster than eight pies. It's not even a contest if you also need some I/O, shared memory, or heavy SIMD.
The video in the OP showed 8 pis, and assuming 4 cores each that's 32 cores total.
A raytracing benchmark would be interesting because you could divide the work up-front and not have to worry about communication between nodes in the cluster, and each node doesn't need that much memory.
Maybe the ryzen 2700 beats the 8 PI cluster, but clearly there's an X number of PIs that will beat the ryzen. Maybe it's X = 10 PIs, or 20, or 200? Idk, but it could also just be 8. There's no way to know for sure without a benchmark.
As always, a benchmark is the only thing that will prove it.