Agreed, in the sense that comparing your own A1c to the chart or anybody elses has to be taken with a grain of salt. Just the fact that you are comparing human beings means that there is going to be considerable variation from person to person in the relationship between A1c and avg BG.
I gave up trying to make the comparisons based on those charts match what I'm seeing personally for my own A1c to BG comparisons. Testing practically 24 hours day and comparing CGM numbers tells me that my own average BG, worse case scenario, is at least 10 points lower than what my A1c would indicate based on charts.
I still find the knowing my A1c from month to month meaningful, and I still shoot for normal BGs and an A1c in the normal range, but whether that A1c is 4.8 or 5.8 is only meaningful for myself and, alone, doesn't tell me much about my control.