Yes further development in NAND flash and in PCIE interface since Optane's production stopped have made it so M.2 NVME can now exceed Optane in terms of sequential read/write. However to this day Optane is still a lot faster than NAND in low queue depth random read/write. Here's the difference with my C drive (a Samsung 990 Pro) with the Optane based cache turned off and on:
Off:
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On:
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So when accessing lots of small files constantly, as an OS (and AI) does in it's operation an Optane cache makes it much faster.
With AI model it would be interest to test out. If it uses say 700GB of memory to fit the whole R1 671b model but at any one time it's only reading/writing to a small subset of that 700GB than a two tier memory with Optane as the bigger slower layer would work well for a "consumer grade" setup. But if it's constantly reading/writing the entire dataset all the time then Optane as memory would be very slow.