They ran that analysis in the study and concluded: “Notably, the effects of willpower theories on everyday self-regulation and on GPA did not arise because students with a limited theory had lower trait self-control. The patterns remained significant controlling for trait self-control”.

You can read the full paper here: https://pdfs.semanticscholar.org/8a69/6afa3b4e82727764f26a95d2577c85315171.pdf