In reality FFMI isnt on a straight line basis so an FFMI of greater than 25 is very much possible naturally above 9-10% bf because your anabolic threshold is greater than when you're around 6-7% and having to remain somewhat catabolic to maintain those levels. My FFMI is about 26.5 but i maintain 11-12% bf with a 5-6000 calorie diet.
I've done one big cut in my life a couple of years ago where i cut to 7% and as a result my FMMI fell to 23 because of losing so much muscle in the pursuit of leaning out as a natural and not having the drugs to sustain anabolism at lower caloric levels. I do very much agree with you though people massively overstate their stats.