That has nothing to do with DRAM; that would be completely obvious if you stopped to think about the cache sizes implied by writing at 5-6GB/s for tens of seconds before speeds drop. Nobody's putting 100+ GB of DRAM on a single SSD. You get at most 1GB of DRAM per 1TB of NAND.

What those graphs illustrate is SLC caching: writing faster by storing one bit per NAND flash memory cell (imprecisely), then eventually re-packing that data to store three or four memory bits per cell (as is necessary to achieve the drive's nominal capacity). Note that this only directly affects write operations; reading data at several GB/s is possible even for data that's stored in TLC/QLC cells, and can be sustained for the entire capacity of the drive.