No they don't. They say that the Secure Enclave participates in the secure boot chain, and in generating non-exportable keys used for secured transport. It reads to me as though user devices will encrypt requests to the keys held in the Secure Enclave of a subset of PCC nodes. A PCC node that receives the encrypted request will use the Secure Enclave to decrypt the payload. At that point, the general-purpose Application Processor in the PCC node has a cleartext copy of the user request for doing the needful inference, which _could_ be done on an NVidia GPU, but appears to be done on general-purpose Apple Silicon.
There is no suggestion that the user request is processed entirely within the Secure Enclave. The Secure Enclave is a cryptographic coprocessor. It almost certainly doesn't have the grunt to do inference.
Nvidia's GPUs are complex. They have a lot of dedicated, multipurpose acceleration hardware inside of them, and then they use CUDA to tie all those pieces together. Apple's GPUs are kinda the opposite way; they're extremely simple and optimized for low-power raster compute. Which isn't bad at all, for mobile! It just gimps them design-wise when they go up against purpose-built accelerators.
If we see Apple do custom Apple Silicon for the datacenter, it will be a pretty radically new design. The first thing they need is good networking; a full-size Nvidia cluster will use Mellanox Infiniband to connect dozens of servers at Tb/s speeds. So Apple would need a similar connectivity solution, at least to compete. The GPU would need to be bigger and probably higher-wattage, and the CPU should really emphasize core count over single-threaded performance. If they play their cards right there, they would have an Apple Silicon competitor to the Grace superchip and GB200 GPU.