Are they actually shooting in 70mm?
No, they are using the new Arri Alexa 65, the 6.5K digital equivalent of a 65mm film camera. This is the third film to use this camera, but first to be fully featured. IMAX is also using the Alexa 65 as a basis for their new digital IMAX camera.
What does this mean for aspect ratio? I kind of liked the idea of the non episodic films having a completely unique visual style, even down to the AR