How can brisket have separable lean and fat, when it is trimmed to 0" fat?
Lean brisket and fatty brisket refer to two different parts of the cut. 

Lean is also known as the flat. 

Fatty refers to the point, a separate fatty muscle group that sits on top of the flat. 

These two cuts are often served separately in American barbecue. Also, because they vary in fat content and therefore tenderness, they are often cut to different widths. 

I’m not sure that this is a perfect answer because I’m not sure I completely understand your question. But I think this information may be important to help you reach a conclusion. 

Please refer to the annotated photo below courtesy of Eater. Especially in reference to the thick layer of fat that rests above the point and how the muscle of the point has a different appearance than the flat. 



To more directly answer the question, the part where it is talking about a fat measurement is literally the thickness of the fat layer. Where it talks about “lean” vs “lean and fat” I think it’s talking about which part of the brisket. The flat or the flat + the point. That’s my interpretation just from an initial reading.