I didn't like the call, but I don't think it's as bad as it first looks. It's pretty close imo. One thing that's overlooked (or perhaps not generally known) is the success rate of such an unexpected onside kick. I can't say I know what it is, but I suspect it's pretty high. And you'll note that it was poor execution that prevented them from recovering (properly).
Probably the main reason I didn't like the call is that I suspect Tomlin would have just run, run, run, punt anyway had they gotten it. In order to make it close to correct, you have to continue to run a reasonably aggressive offense, often going for it on 4th down and even trying to score. I highly doubt he had that in mind.