Um... he used flak against what sounds to me like a bomber heavy fleet. Makes sense the flak should win.
Also, "Jumping in with his ankylon and dunov and hoshikos" So it's not Flak on it's own dude. It's the combination of a supporting fleet with ships that are supposed to do bonus damage against the bombers (which was probably the Advents main dps) that sounds like it got the win.
Also, 12 factories and 2 starbases with factory upgrades on one planet? Sounds like the guy deserved to win if he managed to pull that off.