"Only" 2048 CPU cores per rack is actually not that much by nowadays standards - its 16 U of 2x 64 core CPUs. Perhaps is could be more U if they used the lower core counts but e.g. higher frequency per core SKUs but I don't think they do. (And the picture kind of confirms it). They use 2U servers though so they are able to use lower speed but bigger fans and have more expansion cards and 2,5" form factor drives perhaps. The of course have to fit storage, which needs lots of CPU PCIe lanes for all the NVMe storage and networking (probably 2 or 4 U) and power conversion to power the bus bar and more somewhere. They probably use the 42 U+ standard 19" racks to fit in standard customers DCs. They also don't have such a high power budget as custom DCs for cloud providers do.

1 PB of flash is quite a bit but you could get perhaps 5x as much with HDDs probably (even with a relatively low density of 40x 12 x 12 TB). The problem really is I think, they wouldn't be able to write the HDD firmware in Rust in time (or at all, because no HDD manufacturer would sell an HDD to them without making sure their proprietary firmware is used). SSDs don't necessarily have this property as they are much more like the other components of a modern server.