In terms of why the company decided to open-source it, "We wanted to simplify Atomic’s licensing and reduce barriers as we work with our partners on the technology. There was thought put to “dual licensing” the code under separate commercial and GPL licenses. However, we believe the permissive MIT license brings the most value to Atomic technology end users, including us, THUNDERBEAST GAMES LLC. We also view the MIT as a modern software license being used to develop amazing technology across industries, in games alone we’re in great company with cocos2d-x, Godot, and of course Urho3D from which Atomic was initially forked in the fall of 2014!"
More details on the open Atomic Game Engine at AtomicGameEngine.com.