Garbage time means that barring some kind of fluke you are likely to win. It doesn't mean that there is no chance of winning. Until the clock goes to 0, there is almost always a chance of a fluke comeback.
The Colts scored a TD and then needed to recover the onside kick, march down the field in little over a minute with no time outs, and then convert the two point conversion. It most likely never going to happen. There was like a 3% chance of that happening. That is garbage time.