IMO its more nuanced. They're likely in production ramp-up of the M5 Ultra Mac Studio, for release in the next ~3 months; they have pre-purchased bins of memory from the supply-constrained major memory supplies; and they need as much as they can get because they want to push an M5 Ultra config to 768gb to continue the "you can run local models" story that the M5 Max Macbook Pro started telling last week.
Going beyond 512gb and into 768gb memory is something of a threshold that will allow Apple to claim local capability for significantly more models. Qwen3-235B, Minimax M2.5, and GLM 4.7 could kind of run with no quantization on 512gb, but they'll comfortably run at 768gb. DeepSeek-V3.2 and GLM 5 may also work at some level of quantization.
Going beyond 512gb and into 768gb memory is something of a threshold that will allow Apple to claim local capability for significantly more models. Qwen3-235B, Minimax M2.5, and GLM 4.7 could kind of run with no quantization on 512gb, but they'll comfortably run at 768gb. DeepSeek-V3.2 and GLM 5 may also work at some level of quantization.