tl;dr

The memory bandwidth isn't magic, nor special, but generally meaningless. MT/s matter more, but Apple's non-magic is generally higher than the industry standard in compact form factors.

Long version:

How are such wrong numbers are so widely upvoted? The 6400Mbps is per pin.

Generally, DDR5 has a 64-bit data bus. The standard names also indicate the speeds per module: PC5-32000 transfers 32GB/s with 64-bits at 4000MT/s, and PC5-64000 transfers 64GB/s with 64-bits at 8000MT/s. With those speeds, it isn't hard for a DDR5 desktop or server to reach similar bandwidth.

Apple doubles the data bus from 64-bits to 128-bits (which is still nothing compared to something like an RTX 4090, with a 384-bit data bus). With that, Apple can get 102.4GB/s with just one module instead of the standard 51.2GB/s. The cited 800GB/s is with 8: most comparable hardware does not allow 8 memory modules.

Ironically, the memory bandwidth is pretty much irrelevant compared to the MT/s. To quote Dell defending their CAMM modules:

In a 12th-gen Intel laptop using two SO-DIMMs, for example, you can reach DDR5/4800 transfer speeds. But push it to a four-DIMM design, such as in a laptop with 128GB of RAM, and you have to ratchet it back to DDR5/4000 transfer speeds.

That contradiction makes it hard to balance speed, capacity, and upgradability. Even the upcoming Core Ultra 9 185H seems rated for 5600 MT/s-- after 2 years, we're almost getting PC laptops that have the memory speed of Macbooks. This wasn't Apple being magical, but just taking advantage of OEMs dropping the ball on how important memory can be to performance. The memory bandwidth is just the cherry on top.

The standard supports these speeds and faster. To be clear, these speeds and capacity don't do ANYTHING to support "8GB is analogous to..." statements. It won't take magic to beat, but the PC industry doesn't yet have much competition in the performance and form factors Apple is targeting. In the meantime, Apple is milking its customers: The M3s have the same MT/s and memory technology as two years ago. It's almost as if they looked at the next 6-12 months and went: "They still haven't caught up, so we don't need too much faster, yet-- but we can make a lot of money until while we wait."