The Valar don't know everything. And even if they did they wouldn't have the power to control as great a thing as the fates of Men. Eru made Men and Eru (maybe) made Fate. But Men are not yolked by fate simply because they are not of this world. When it says that Men are not bound to Arda it means that Men aren't bound to Arda's Fate. And they aren't bound to Arda's fate because they are not of Arda itself. They do not live or die with Arda as Elves do.

Their actions also are not constrained by Arda's fate because they will be leaving Arda and never coming back. While Elves are forever bound to Arda and whatever they do will effect their lives directly. Men do not have to suffer from their actions of destruction to Arda. Maybe their descendants will, but not themselves.

As for how this passage relates to it. Well, it just means that the Valar do not know everything about the fates of Men. And that Men indeed are not of this world. Since the Valar are more close to Arda, I think it would explain why the Valar treat Men so differently from Elves. Elves are the ones that are bound to the world they made with their song, while Men are just comers and goers.

For more about the fates of Men take a look at the debate of Finrod and Andreth in Morgoth's Ring. It'll tell you a lot.