I've heard that before, not sure where...but I think that's bullshit.
For example, someone like Francis Ngannou. Let's pretend he's natural because there's a logical reason and good chance he is (dirt poor, from Cameroon, Africa).
Plugging him in at: 6'3, 255, 12% BF = 28.11 on the FFMI calculator.
Let's pretend he's actually 6'4, well that comes out to 27.37.
Now at 15% BF, it's 26.44.
So if he's actually 6'4 and is actually ~15% BF at the confirmed weights of ~253-260 which he's weighed in at, then he's still reaching that very few, elite range or is a bit over it technically. Makes sense, he's obviously genetically gifted. But I think there's other guys like him at varying heights that can pack on muscle and be relatively lean naturally. I don't think the FFMI, much like BMI, is not an accurate tool and it's kind of some psuedo-science bullshit.
I think a ton of athletes are on steroids/PEDs, specifically in the NFL and olympics, probably in MMA and even under USADA, but I also think there's a counter-warped perception of what is achievable naturally. Not everyone is going to look like Francis Ngannou obviously, or even say Eric Bugenhagen (lol) but I think it's very plausible each guy is 100% natural. Wouldn't floor me if both guys were on steroids either though. I just think FFMI 25-26 as some scientific law is stupid, and I've heard it before...can't recall where.