I am not an expert(caveat). If this is referring to ADP as a Professional employer organization(PEO), then the question becomes "can I avoid being an employer if I use a PEO?"
I would question the avoidance by the company(you) of being classified as an employer, since sole proprietor's may not pay themselves wages.
This PEO told you yes, because THAT'S THEIR JOB, and that's how they make money. I would not take income tax advice from a payroll company, any more than taking income tax advice from a financial planner.