Bit of both - if you're arriving with real world military training then there will be bits where you already know the subject matter, but there will also be parts where it is completely Arma/4IB focused. You have to remember that this is geared at the lowest common denominator - meaning it's aimed at someone who potentially has no military knowledge at all, and no Arma experience either, so it has to cater to those requirements.

In my experience, I found that even though I knew the fundamentals of what they were teaching (section attacks, formations, etc), I still came away having learned something - the instructors and the course are full of useful information regardless of your previous experience, so it's definitely can't be considered a waste of an evening.