It's 410 GB/s peak for DDR5. The "up to 800 GB/s sustained" is for GDDR6 and POWER10 isn't slated to ship until Q4 2021 so it isn't really a direct comparison with hardware that was deployed in 2019.
IIRC, the new memory connection they introduced with the latest POWER9 chips traded bandwidth and physical distance for a bit of latency.
This POWER10 chip under discussion has 1TB bandwidth to devices with expandable RAM.
Yeah, I didn't think it was possible. But... congrats to IBM for getting this done. Within the context of this hypothetical POWER10, 1TB bandwidth interconnects to expandable RAM is on the table.