YMMV.
One thing to remember is that "Lung Cancer" is not just one thing. There are mutations of different genes, there are overexpressions of different genes. Each one has its own new medicines. Also, some peoples' cancers are more aggressive than others, and while for many they can find the right drug, for some nothing works.
Keytruda works wonders for some. It did not for me. I had 4 treatments of the CPP (carboplatin, pemetrexed, and pembrolizumab) triad, which had some success. They then put you on "maintenance," which is the PP without the carboplatin (which is the really old school platinum-based chemo). Maintenance did nothing for me. My main tumor grew more than 50% in 2 months.
Last summer I started 9 months on a chemo/immuno that was geared towards my specific mutation. It actually did wonders. It resulted in a 98% shrinkage of my main tumor before I ended up with pneumonitis from it and had to stop. But I've been able to be off treatment for the entire summer. I know there are others who have tried this, and it didn't work.
So yeah, I'm really, really glad your relative was able to get some relief from the Keytruda. But I also wish it were the wonder cure for everyone that it was for them.