It's not a CAA requirement, it's a company requirement. If he has a CPL he can fly a airliner legally according to civil aviation regulations.
You apply for a job as a civil engineer building bridges etc the company is gonna say we need a B.Sc (Civil), professional registration with SAICE and we need you to get an M.Sc Eng within time frame X. He meets the legal requirements as a Pr Eng so legally he can do the job and if a bridge he designed collapses things are covered even though they discovered he falsified the M.Sc as that is not a legal requirement to perform the job.