What they may mean by soft is that, apart from warping, the metal does become soft. I have seen a soft head and it usually happens if the head has been done a few times. What seems to happen is that the head can not take the pressure from the head bolts and will get squeezed by them. This just makes matters worse and you may need to do the head again. Let them test it to see if it's soft, if they say it is and your still not satisfied or sussed, get a second opinion. But they do become soft after time and after a few welds, quite a few!
It's just a like a piece of flat metal, the more you bend it the softer it gets. Best explaination I can give.
"Give me any task to do, and I will find something else to do in it's place." Plato - a definition of procrastination