Yes, Trivium, Lamb Of God, etc have played before, but they've been exceptions on the bill, there has never been a nu-metal band play BOA, the closest thing was Soulfly years ago.
Actually there have been a couple of nu metal bands playing before, its just most of them have been on the lower stages so noone has noticed. I remember Father playing in 2012 (I think), who opened up with a medley of nu metal covers. They were actually pretty decent.
As for the announcement, its yet another one that will definitely attract all the naysayers and those on the fence. They must have had real trouble attracting any big names this year, which is strange considering how many seem to be touring.