Here in the UK, the Law tends to regard any assault with punches by a professional boxer, as "assault with a deadly weapon". Well, that's the reading from past court reports in print.
Gary Delaney ex-LHW and HW boxer is in prison. He was a doorman who punched a customer, who fell over and banged his Head on the pavement and died. The same occured with Kostas Petrou ex-MW. The Law takes a dim view and expects higher standards and discipline from boxers.
Otherwise, the OP is correct, it's myth etc.