This statement is true.
A contingency fee is a method of paying someone ( for example a lawyer or a scientist ,) in which no money is paid until a particular aim is achieved. In other words, the lawyer will only get paid if he wins the case.
Mark is a forensic scientist. To agree to be compensated for his work through contingency fees presents him with a potential conflict of interest and work ethics. This should be avoided in his profession and it is marked as a guiding principle to be obeyed.
Mark would have a financial incentive to provide a forensic opinion that is favorable to his client ( only then he would be paid for his work). This is seen as a threat to impartiality, the forensic may use the evidence in a way that favors the positive outcome of the case. This is strictly prohibited as all medical ( forensic ) opinions should be based on facts only and not to be influenced by any kind of reward or personal gain obtained through them.