The default game engine (which we call "Superpowers Game" because we're very creative with names :D) uses Three.js in WebGL mode for both 2D and 3D. There isn't really a deep distinction between 2D and 3D in this case, except for using orthographic cameras. So no, games made with it wouldn't run with just Canvas 2D support.
A Phaser-based project type would, though. we've started putting one together, should come out fairly soon after we open source!