The issue I have previously seen with this is the folding assembly (for instance, the LAW folder) potentially made the brace length-of-pull too long and thus it "may" be considered a stock. Now, I don't know how any of that shakes out with the ATF now and what their rules are.

None of this is to say that's the rule, was the rule, or even makes any sense. That was simply the issue that I ran across when looking at this very question previously. Ultimately, the point is the folder concept wasn't an issue, but instead the mechanism of the folder made it an issue, especially with an SBA3 or SBA4