Misinformation, I imagine. The CFPB has just published a
proposed rule that would mandate specific content and arrangement for periodic mortgage billing, but the accompanying information says "The periodic statement requirement generally would not apply for fixed-rate loans if the servicer provides a coupon book, so long as the coupon book contains certain information specified in the rule and certain other information is made available to the consumer. The proposal also includes an exception for small servicers that service 1000 or fewer mortgage loans and service only mortgage loans that they originated or own."
Note that the exception for coupon-book servicers only applies to fixed-rate loans.