Sorry, my question is really about the lack of accuracy of the distance away.
Why is the diameter so relatively accurate compared to the very imprecise “about 25,000 light years away”?
Sorry I didn’t check this over better.
I think you mean precision and not accuracy. Accuracy is a moot issue. The precision comes from calculation of the gravitational effect of the black hole on surrounding stars. You could argue the precision is too significant, but as long as the methodology is stated in the paper and everyone s on board with the method, it becomes an acceptable estimate. You could also formulate your own methodology and publish your results, which is how science is supposed to work.