Dummy "it" Usage

Dummy "it" Usage

The dummy "it" is used in the following transformation:
it + [PREDICATE] + [SUBJECT]= [SUBJECT]+ [PREDICATE]
An example would be:
It [is a tragedy] [what happened] =[What happened] [is a tragedy]
But then I found this:
The MLM business model has always had its detractors simply because the
math usually doesn't hold up under scrutiny. Having to constantly recruit
new salespeople below you in the food chain creates a situation where you
run out of people in the global population who can sell your products.
It's a fine line between a pyramid scheme and a self-sustaining operation.
If I use the dummy-it transformation on the bold-italic part:
It [is a fine line] [between a pyramid scheme and a self-sustaining
operation] = [Between a pyramid scheme and a self-sustaining operation]
[is a fine line]
But, the part "between ...." is not really a subject. It seems to be part
of the predicate "is a fine line". Could the original example be poorly
written?