SM-6 has been known to have some anti-ship capability for almost a decade, so nothing new. To get any sort of range it is probably using a ballistic trajectory or something similar, which means its probably not too difficult to detect compared to an actual sea-skimming missile, and even if it hits the ship, SM-6 has a tiny 64 kg warhead. For comparison, HHQ-9 also has a anti-shipping capability, but with a ~150 kg warhead. I doubt either missile is used like that though, considering how valuable magazine space is on those ships.One thing to consider is what threat does the SM-6 pose in an anti-ship role? That missile is a relative recently development and its anti-ship functionality was only recently revealed. As far as I am aware, the US Navy still posses SM-2 and SM-6 so using SM-6 in an anti-ship role is totally viable and they have a large stock of those weapons. SM-6 gives them a high supersonic long range anti-ship missile while is deployed across the fleet. The US Navy is not just limited to Harpoons and Naval Strike Missiles.