I'm largely responsible for this, so I'll try to answer.
Technically it's not NAT64 today. Different prefix for one, but it's also not translated at the IP layer (yet). For TCP, we terminate the TCP in tailscaled and make a new TCP connection out and switch them together, so packets are not 1:1 end-to-end.
We also had grander plans for the 32 "site-id" bits in the middle there. Instead of just a 8-bit (now 16-bit) "site ID" number in there, you could actually put the 32-bit CGNAT IPv4 address of any peer of yours, and then access its IPv4 space relative to that node, without any configuration.
Say you have an Apple TV plugged in at home.
Then you're at a coffee shop and want to access something on your LAN and don't have a subnet router set up.
You should be able to `ssh 10-0-0-5-via-appletv.foo-bar.ts.net` and have MagicDNS map that "appletv" as the "Site ID" and put its 32-bit CGNAT address in, and then parse out the 10.0.0.5 as the lower 32-bits, and then have Tailscale route your packets via your home Apple TV node.
All subject to ACLs, of course, but we could make it a default or easy-to-enable recommended default that you could do such things as an admin for your self-owned devices.
So why it's called "4via6"? That was just kinda a temporary internal name that ended up leaking out to docs/KB and now a blog post, apparently. :)
I never said it didn't work with UDP or ping. I described what it does differently for TCP.
Anyway, I'm sorry we offended you with its name.
I personally think it would've been more offensive to use an existing spec name and then not implement the spec of that name perfectly. (which is likely if our needs/goals only 90% overlap with the spec we pick)
At least if we screw up this implementation, we didn't tarnish anybody else's spec or its name.
As far as I understand it, both involve translating between IPv6 and IPv4, but NAT64 is a broad standard for general IPv6-to-IPv4 internet access, whereas Tailscale's 4via6 is more specific feature to solve a niche problem of overlapping private IP ranges within a Tailscale VPN environment using some proprietary addressing scheme. But it's been a while since I was deep in network land.
Most people working outside the network layer are not familiar with the basics of IPv6 and how it interops with v4 systems. In fact, I would bet that many AWS admins are not familiar with dualstack VPC configurations, for example. This product name communicates clearly to those users what the value prop is.
