The most significant aspect to highlight in this respect is that the results of polygraph tests are not always accurate. There are lots of cases when guilty individuals have been able to defeat a polygraph test successfully. What is more, there are also people who have been wrongfully convinced because of their polygraph test results. The reason why that happens is that such individuals are simply anxious that they have to take a test and not because they have committed a crime. In case the examiner and the person taking a polygraph test have had a tense interaction prior to the test, there is a very high chance that the results might be inaccurate.
As you have already understood, what it means is that the result of a polygraph testing alone is not enough to be 100% sure that the person is guilty. That is the reason why it is so significant to run a DNA test in case it is possible as well as examine all available evidence closely.