0.5 GB of SRAM is exactly what the Cuckatoo32 [1] Proof-of-Work scheme needs for optimal solving efficiency. Solutions are 42-cycles in random bipartite graphs of 2^32 edges on 2^32 + 2^32 nodes, which can be found by repeatedly removing edges that have only one adjacent edge. Tracking which nodes have incident edges takes 2^32 (completely randomly accessed) bits which is exactly 0.5GB. Another 0.5 GB is needed to track alive edges but those can be accessed serially for which DRAM suffices.

[1] https://github.com/tromp/cuckoo

The Intel D270, Nvidia DGX Spark, and (somewhat) AMD Strix Halo also use the same approach, so it seems to be getting pretty common.

SRAM wise it doesn't seem too insane. About the same ratio of memory:SRAM as the 5090 (IIRC). Still more than I would have expected. I assume they found it helps process the current layer faster or something.