That's specifically excluded under this rule. Their worst nightmare is having AR uppers AND lower be considered the 'receiver'. They want to codify that the lower is the receiver to get them out of the court cases that have been hammering them lately.
The marking section is mainly aimed at "Personally Assembled Firearms" (80% guns) and how they have to be treated if, and only if, they are sold to an FFL or transferred through one (nice tie-in with Universal BGCs, huh?).
Good. I have not had time or enough to drink to read the thing. Thanks for letting me know.