Technically CloudFront requests to the origin server are IPv4 only, so there's still IPv4 in the stack even if the distribution has IPv6 enabled.

Personally this is one area I'd like AWS to focus on, along with load balancers listening on IPv6 only (currently it's dual stack). Every load balancer we deploy takes up at least 3 IP addresses - I'd quite happily switch them to being IPv6 only if CloudFront could access them over IPv6.

No, you're talking about API Gateway triggering lambda. My understanding is that is is about the invoke endpoint being listening on IPv6, so you can invoke your functions from your cli or SDK using IPv6.

I would not say putting a CloudFront in front of Lambda is a good practice anyway. It highly depends on your use cases (load, frequency of data changes, etc). In general, I probably lean the other way and use a CloudFront only when necessary as it only introduces additional architectural complexity which makes it more difficult to troubleshoot issues. You can also get in a bad place if there are two many CloudFronts in the call stack.