Well they're specifying the AMD EPYC and one of the things that the server line of AMD CPUs do that the consumer grade ones don't, is they have lots of connectivity. So for example an AMD EPYC 8324P is a 32 core CPU with 96 lanes of PCI Gen 5. Given that the 4090 GPU is PCI Gen 4, I think that's where you get the discrepancy. The 6 GPUS are connected in parallel to the CPU with 6 x16 connections (96 total lanes), the CPU could do this at Gen 5 (64GBs for each GPU) but the 4090 GPU is Gen4 only, so you'll only actually get 32GBps per connection.