Rather stupidly, the "Final Rule" mentions "factoring" despite eliminating the factoring sheet for their own regs. This dichotomy was brought up by any number of people in the public comment period and was never addressed by the agency before pressing forward with the enaction. This has al lthe appearances of a blatant APA violation, and may be an issue in any number of pending lawsuits, which may explain the relative "silence" on the topic.
BATFE has stated, just not for the record, that anything that changes a hand gun for "use by just one hand" to "use by two hands" is, per le, an SBR, and this, illegal without a Form.
Does adding an angled foregrip to a pistol make it verboten? Dunno. Until some one is arrested for it, no body much knows. Which is not how US law is supposed to work.