Can't comment on the stats used to derive the figures, but presumably someone calculated that a failure rate of 1x10^-5 / hour is pretty darned safe, hence it was used for the study.
They really cannot then say "yup, piston engines fall within our pretty darned safe level, but not by far enough".
Tantamount to "We've placed a stop sign here, but you must stop 50m short of it or we'll ban you from driving".
Also, a nice statement in there: "apply the same standards in the same manner". Might be nice if they applied their own standards.