It would be 273 GB/s (gigabytes, not gigabits). But in reality we don't know the bandwidth. Some ex employee said 500 GB/s.

You're source is a reddit post in which they try to match the size to existing chips, without realizing that its very likely that NVIDIA is using custom memory here produced by Micron. Like Apple uses custom memory chips.

Yes, but for the price of that single M3 ultra I could have 4 of those GB10's running in a 2x2 cluster with the full NVIDIA stack supported (which is still a big thing)

So M3 preference will depend on whether a niche can significantly benefit from a monolitic lower compute high memory vs higher compute but distributed setup.