Even if there was [which there isn't] DC power coming out of the BB DECA, by connecting it to the red port of the splitter, this wouldn't block anything since this is the power passing port.
Our local trainer has always taught us to put a 2 way splitter inline with broadband DECA units, it was also in the DirecTV training video, (which you can watch if you have a password to satinstalltraining.com). Anyhow the input to the 2 way splitter comes directly from the SWM 4 or 8, hook the BB DECA to the red port the second output can be used for an IRD.
Do you have a link to the training video?
While I don't always agree with them, I can't honestly believe this is what they're saying/showing.