We demonstrate that, for certain array configurations in free space, traditional MIMO theory leads to eigen-modes identical to the OAM states. From this we conclude that communicating over the sub-channels given by OAM states is a subset of the solutions offered by MIMO, and therefore does not offer any additional gains in capacity.
In other words, OAM is a perfectly legitimate technique for encoding data. It just happens to be a subset of what's already capable with MIMO. It's also worth noting that your paper discusses radio waves, and the OAM demonstration discussed in the submission is in the optical. There may be limitations that prevent certain MIMO techniques from being applicable to optical transmission, especially in guided-wave situations (i.e. optical fiber). In fact, the research group I worked with during my Ph.D. was looking at encoding extra information on single photons using OAM to increase the data capacity of quantum communication networks, a situation where MIMO is almost certainly not applicable.
It's worth noting that Alan Willner is no nutjob. He's worked at Bell Labs, has a chaired professorship at USC, is a fellow of IEEE, OSA and SPIE, and is an editor-in-chief for several reputable academic journals (JLT, Optics Letters, JSTQE). I had the pleasure of working with him on an unrelated project 5 or 6 years ago, and there's no reason to believe that he's trying to pull the wool over anybody's eyes. There's certainly no professional reason for him to do so, his CV speaks for itself: http://csi.usc.edu/faculty/willner.html