Salary cap doesn't apply to playoffs. So, no cheating.
You might contend that Kuch intentionally got hurt and unnecessarily had hip surgery, or that he changed the date of his surgery to help unburden the Lightning payroll. I'm guessing a competitor like Kuch would not do that, but it's possible. The league did have a look at all of it and said no foul.
One interesting point. This little loophole with the salary cap has been around for a while. Blackhawks did almost the exact same thing with Patrick Kane in 2015. Kane was injured and unable to play most of the regular season, but entered the playoffs (and scored 8 goals). Blackhawks won the Stanley Cup that year, at home, against the Lightning. One of the NHL teams requested that the NHL change the rules regarding LTIR (long term injured reserve) guys. There was a vote, and the entire league except two teams decided not to change the rules - the voted to keep this loophole open.
The team that asked for the loophole to be closed? The Tampa Bay Lightning. So unless you're with one of the teams that voted to get rid of this loophole (Blues and Lightning), qft.