That is a good explanation (contrary to the others). The important part is not that Dayne isn't wielding Dawn, or even that one KG is missing. The important part is that Dayne wiped the floor with Ned's allies. He still overpowered them completly. Dawn, Dual wielding, or toothpick it doesn't matter.
As you stated, book readers should be prepared to be disappointed. In books fight are never the same as TV ones, because it isn't the same transcript material. What works in books doesn't always work on TV and vice versa.