> The immune system could kill the mutated cells and thereby promote the unmutated ones.

This happens all the time. The mutated cells we see are the ones that immune system couldn't detect and kill. Fortunately they are still overwhelmingly non-cancerous, but unfortunately some might be.

Yes I agree, I was just responding the article's "“We cannot reverse a mutation once it occurs, ..." I don't think that is entirely accurate. Also, I think it is a dynamic process, so even cells the immune system hasn't killed yet could be found later. Or the mutation could cause other deviancies that will make the cell uncompetitive with healthy cells. But it is a slow process - it takes years for former smokers' lung cancer risk to return to near that of never smokers. And it probably never gets there - some mutated cells may never be detectable and there's clearly also a threshold beyond which the cancer is irreversible, at least without intervention.