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Credibility should lie with the authors of the study - graduate students who probably do know what they're talking about and could probably say the plural form of "vortex" if they thought about it long enough.
And as the vorteces leave the wingtips and begin to dissipate, there is one side flowing "up"-the wing side and one "down"-the fuselage side. At 3-5 miles, I suppose they're weak enough to not interrupt the flight characteristics significantly. If you notice, the third plane flies in the middle of two planes ahead, so both of the "up" sides of their trailing vorteces are in the middle of the chase plane's flight pattern - probably yielding the better stability they're talking about.
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