The example is used for standardisation purposes, showing that depeninding on bone structure, 2 people can have an FFMI of 25 while 1 has an upper arm of 14.3 inch and the other has an upper arm of 19.1 inch.
Even though both are equally muscular, generally, the 14,3 inch arm will be regarded as laughable while the 19.1 inch arm will be met with praise.
Since both people are equally muscular, and from an FFMI standpoint as you indicate, the thinner guy might even have an FFMI above 25 (but still with arms below 15 inch) indicating he is even more muscular then the bigger guy.
Conclusion, absolute numbers say nothing at all and are pointless. Aside from the numbers, the overall physique is what counts and thus, the lower BF% are what count and are the goal. As a lean physique will bring out the muscularity and needed aesthetic shape that will make a trainee look larger then he actually is.
It is a game of illusion in which one should carefully study onseself to assess the weak and strongpoints first. A trainee, for example, boasting small, narrow shoulders... should emphasize a small as possible waist and train especially the lats and side delts to increase shoulder widht while getting as lean as possible to decrease waist size.