But there is the possibility that the demodulation/modulation methodology is easier to implement than other fast modulation schemes.
This is the key point. MIMO schemes require a lot of complexity in the receiver, which is workable at Wifi/LTE frequencies of a few GHz where you can get electronics to reliably function. Here he's working at millimeter wave frequencies (30GHz to 300GHz) were electronics either doesn't function, or is extremely expensive. You could think of this work as a MIMO-like spatial multiplexing scheme that doesn't require complex processing in the receiver, and is therefore implementable at very high frequencies.
As others have noted it's a separate question whether and how this could scale up to real environments. What elicits research interest in millimeter waves is the potential for much higher bandwidth than LTE is capable of, plus the fact that millimeter waves can propagate a kilometer or so through air. So although it's still futuristic, one can start to imagine what a millimeter-wave wireless communications network would look like.