I don’t think the 2011 talk was referring to v6, maybe it was. But Prerouting not = input + forward. Look at the 3rd image here. The reason I use pre and post on DNS traffic is because the forward chain is not able to mark traffic sourced from the router itself and also traffic coming into the router from LAN.
Maybe there are other ways of accomplishing the same thing. Perhaps your question will reveal a better way. At the moment, this is all I know. However, when I was watching traffic from the connections menu, doing what I’m doing now was the only way to see it show up marked.