The thing is that even if the monitors used are not fully correct measuring-wise but the mixing engineer finds them to sound right with well-known reference tracks, he still will be able to make correct mixing decisions with the target of those reference tracks. But if he instead uses some other monitors he doesn't like the sound of, even if they are considered objectively better, he will most likely overcompensate for the things he doesn't like with the sound. He could still use the reference tracks to get in the ballpark of correct, but it's a way better starting point to like how the reference tracks sound on his monitors.
If a professional mixing engineer will be able to work for 8-10 hours a day, it would be a really bad choice of studio monitors if he gets listening fatigue in an hour or two, no matter if the monitors he uses are considered objectively great or not.
Bob Clearmountain is considered a great mixing engineer who's in the mixing world and is also famous for using Yamaha NS-10s, do you think his mixes would have been significantly better if he had used Genelec or Neumann monitors instead? The same goes for all other great mixing engineers out there who are used to completely different sounding monitors of all kinds, and of all types of brands which are all measuring differently from each other. Do you really think their mixes would have been significantly better if they had chosen other studio monitors with objectively better measurements than the ones they are used to and have already given great and proven results?
I don't think so, I think the use of (for the mixing engineer) well-known pair of studio monitors, using well-known reference tracks that sounds right to him on those speakers, and mixing in a familiar studio room will most likely give the best result.