The ROAS numbers given by FB (your employer) are far from being a real estimate of the causal effect of adverts. For example, there's no way to have global holdback against other advertising channels not controlled by FB. Double-digit anything are easy to get on any advertising perfomance metric if you don't scale your spend, so it's not really an argument.

(Disclaimer: I've built multiple bidders & ads incrementality models over the years so I'm probably biased against the "supply" side of ad inventory).