Go back and read the original article - 12% saving at 55 ft. Let me repeat that - 55 FEET! It wouldn't take much of a system malfunction to result in a mid-air at 55 ft. - especially given it would be on auto-pilot so the crew reaction time to a malfunction wouldn't be good.
Airbus has said 1.8 nautical miles - which is doable, but the potential benefit would be a tiny - less than 1% - and things like turbulence and cross winds mean that the 'sweet spot' would be constantly moving around so staying in the right spot for hours on end would be non-trivial.
There may be some potential benefit for military - where close formation flying is the norm.
For passenger service, it's a pipe dream...