The tests are made to measure even very low IQs. They can also be adapted to people who are unable to speak. And there are short forms for those who struggle with their concentration. If there are many cases that are so severe that IQ testing is out of the question, I’m sure many professionals and scientists are aware of this as well as the low measured IQs.
One to two standard deviations is 15-30 points. It’s a bit confusing, but it’s done this way to make comparisons easier. 