Why is the D7000 commander mode flash firing during exposure when the manual says it won't?

I'm wondering if I'm using the D7000's Commander mode incorrectly. Page 225 of the Nikon D7000 manual says

The built-in flash does not fire [in "--" Option], although remote flash units do.

Yet what I've found is that the built-in flash in fact does fire, adding ugly on-camera, direct flash to my photos especially for short-distance subjects. I present Exhibit A below, where it is seen that both the remote SB900's and the on-camera's flash are captured, even in "--" Commander option. How do I fix this?

Answer

They lie.

It does, it just not supposed to fire enough to matter. The flash is how it communicates with external units.

You can get an SG-31R unit to block it and let the IR only through.

Your other option is to ditch CLS and go with radio triggers - of which, if you search, we have various questions about.