For the reason I've already explained, this is not true.
The problem the discovery of proteins that "fix" errors (mutations) presents to us is that this process will have its own rate of change. In the near term it may make little or no difference in the rate of change. Over a long period of time it may well be of vast importance, particularly if the fix rate has a large variance. (NOTE, taking a WAGuess I'm presuming there are proteins produced in the mitochondria that fix "non coding sequences", not just genes).
Our current estimations of the mutation rate of a particular strain of mytochondrial DNA may well be skewed by a previously undetected, but significantly large error in the fix rate.
Like I said, we need grants, we need new buildings, we need platoons if not divisions of grad students.