Correct, but, each marshall's post is manned by a senior post chief and his deputy, either 2 or 3 flag marshalls and a number of incident marshals(number varies depending on the supposed likelihood of an incident at that location), when a report is sent to the stewards of an infringement it is because at least 2 marshals have witnessed the infringement and it is backed up by a written report which contains the signatures of the 2 or more marshals who had witnessed the incident so it doesn't depend on just 1 person.
I'll never forget a few years ago manning the marshals post at the entrance to the new pit lane and writing nearly 50 reports of track irregularities during first practise!
Ian