The rating doesn't take into account the presence of GMOs in a product since disclosing GMOs is not mandatory unless the presence of GMOs is greater than 0.9%. Currently, very few ingredients contain more than 0.9% GMOs.
If the ingredients contain less than 0.9% GMOs, disclosing it is not mandatory and is rarely done. Detecting GMOs requires a laboratory analysis, which is very complex and costly.
However, Yuka does take into account the presence of official organic labels. These labels prohibit the use of GMOs, but still allow the presence of small traces of GMOs.