They're not saying, "commercial jet engines are more reliable; therefore we should use them". The issue is that the Merlin 1C engine simply hasn't had enough fl

They're not saying, "commercial jet engines are more reliable; therefore we should use them". The issue is that the Merlin 1C engine simply hasn't had enough flights to build up its denominator. It doesn't make sense to normalize for thrust here. This is just failure rate. There's nothing wrong with having a comparatively high failure rate when the sample size is several orders of magnitude smaller: the statistical significance is just weaker; we acknowledge that and move on.