That is only 50% true.
With built-in subs, they are always active.

Actually, in Pure Direct Mode 5.1, your dedicated Sub (0.1) is active.
Only in 2.0 Pure Direct is the dedicated Sub NOT active.
See, the reason is I'm not using a receiver or Pre-pro for my Denon DVD-3800BDCI or DVD-5910CI.
In a receiver or pre-pro, Pure Direct is only for 2.0 Channel.
But in the Source Players, Pure Direct can be 2.0 or 5.1.