He hadn’t form.
While Jade and Dave were arguing, he hadn’t form.
If John formed, Jade would have smelled him, stopped what she was doing with Dave, and chase him.
John didn’t want that, whether Dave needed to talk Jade about his powers, or John accidentally lead Jade somewhere else to cause more disaster.
John is literally stopping himself from making that happen, before it could happen.
He is changing the story by changing himself.