I was interested in the number for Russia; 2 out of 4 mobile networks do not provide ipv6, and wired network ipv6 availability depends on provider; provider-supplied routers generally disable ipv6 anyway. So the 78% seems high.

Could be that this is measured from youtube; people use vpns to go there, and those typically have ipv6. That would mean that country is tracked by account, not source ip.