Its a dumb moment in and of itself. The entire planet is coccooned in symbiote goo. You can't hide anywhere, so closing in around the island won't even work. And even if they all warp to Mars, Knull is targeting all life. He'll just target Mars. They could maybe flee to Hell or the Otherworld, but it would require abandoning Krakoa.
Like I said, this is the 'aliens will never come here!' moment in an action movie. Or something like the walls from the first Pacific Rim movie.