The more I think about it, the more I think the 300GB/s number is just Nvidia and partners rounding up.
Could be, or they could be using faster memory. Our DGX Spark units use LPDDR5X-8533:
Size: 128 GB
Configured Memory Speed: 8533 MT/s
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This article from The Verge mentions the new RTX Spark Asus models are using LPDDR5X 9400 on the P16 and LPDDR5X 9600 on the P14. This could be the reason for the ‘bump’ in memory performance advertised for the RTX Spark platform.