To put things in perspective, I'm a 9 and I happen to have a Ph.D. in plasma astrophysics (not terribly useful in real life, but it was fun to pursue at the time). Most 5s would have a hard time demonstrating that I don't understand what I'm talking about in that area of mastery, including all of the complexities. The difference that I have from 5 physicists, though, is that I know how to explain all of this stuff to people who would never have the time to understand the math, at a level that conveys fundamental understanding. Being able to take a complex topic and simplify it is essential to communication, and it's a skill that most 5s (and most INTPs, who are very frequently 5s) tend to lack. The 5s will try to explain every excruciating detail, because each detail is so essential and important to understanding the whole, and without which one's understanding would be incomplete.
Note that most people will have incomplete understandings, and that detailed approach is often insufficient to convey anything resembling expertise with the idea. As an example, when I taught, I ran into plenty of students who could manipulate trigonometric calculus expressions with ease ... but all I had to do was label the TOP angle of the inclined plane problem instead of the typical way at the bottom of the ramp to demonstrate that they didn't have a clue what sin(theta) or cos(theta) meant - they understood complex recipes without understanding the underlying principles of trigonometry. (Not that I'd purposefully humiliate them ... this evidence was gathered from answers to homework problems.) It made me doubt their understanding of why Newton invented calculus in the first place, what calculus is for.
Just as 9s shouldn't equate an understanding of a simplified explanation with full understanding, 5s shouldn't equate an understanding of complexity with a true understanding of the topic as a whole.