eslHQ - Present per./ present per. cont.

The problem lies in the first sentence which should be We are film producers - it's a compound noun with the first noun acting adjectivally, so it can't be plural.

First of all :

One use of the present perfect (the one relevant to your example) is to describe events occurring in past to present time.
Continuous aspect expresses events which are or were ongoing.

When the event(s) are past and completed (so not on-going), the simple form must be used : We've made 16 films since we graduated from college (= 16 individual completed past events in past to present time - since we graduated from college.)

However, if there is no indication that the action(s) described is/are individual events, then the two concepts (past/present time and ongoing events) can be combined to produce the present perfect continuous - which in this case can be used interchangeably with the simple form. So here :
We've made films since we graduated from college.
= We've been making films since we graduated from college.
The two, in this context, have exactly the same meaning. The only difference is that the second emphasises the on-going concept more than the first.

Here's another example:
I've had a headache three times this week (three separate, individual headaches in past to present time - this week)

But either :
I've had a lot of headaches recently
or:
I've been having a lot of headaches recently
(The headaches are presented as an ongoing problem in past to present time)

Possibly the textbook used only the first structure - the present perfect simple - because the present perfect continuous had not yet been introduced. But the simple form is certainly not wrong.