Amdahl's law does not apply to multiple tasks. It's a law trying to show you the gains you may expect from parallelizing a single task, but as soon as you run multiple tasks at the same time you can easily break those borders. A game engine consists of multiple tasks (player input, preloading, physics, AI, sound, scripts, management, feeding the GPU) and can therefore scale beyond Amdahl's law even with lower parallelisation in individual tasks.
All said though, the Arma 3 videos fail to impress me sofar and I can't see where those CPU cycles go. That engine is using a lot of cycles to display a few hills , a jeep and some trees at 30 fps, which is quite bad even for an alpha preview imho.