He might have just grabbed Magneto. And considering how Trask as a Prime Sentinel was zipping around soloing an entire team of X-Men, Bastion could have moved so fast no one would even have been able to see him, even if they were actually paying attention. That battle was raging across blocks and blocks of a city. Where Magneto bunkered down with the Morlocks and where Gambit blew up the wild MasterMold probably wouldn't even be in the exact line of sight. In any case, I'm certain this will be addressed in the final episodes.