That's about correct - a barbell movement that is similar to the main movements: multi-joint, etc. Of course, this is a loose interpretation as you know and can imagine how people will take this entirely out of context and shit all over the bed.
In general though, that is the basic idea: FSL, BBB, BBS, SSL, HH5, SST - there are dozens within the program itself and ALL involve a big movement. Of course, and I think you are smart enough to understand this, if you aren't sure if the movement is a supplemental movement, its probably not. Too many people turn their every lift into a main movement. You see this with chins, pull-ups, barbell rows, dips, dumbbell work or whatever article they just read on a certain lift that makes them change their entire persepective just because some "fitness authority" wanted to get some hits to their website.
Anyway, if in doubt, there is no doubt.
FSL is a part of the program but it's not an important part: the important part is the principles. PRINCIPLES win.