It's just a fan rumor based on Rivera's cryptic comment, so probably not what's going on anyway. But he's 30, was one of the best LBs in the league on the day he retired, and could be an incredible mentor for Davis, not to mention a top tier contributor for 3-4 seasons. It's a no-brainer if he wants to play. The only risk would be he has a couple more concussions and has to retire. I don't see the downside (if he decides the risk is worth it).
London Fletcher had a number of concussions over the years before he came to DC (and a couple here). Not saying that's a good thing - it's obviously not. But its a matter of degree. I'm sure if Kuechly was genuinely concerned, he'd decline a chance to return.