Actually, with many legal things, it's not that clear. Insiders on non-filing pinkies generally don't file. There is debate out there on this issue. Whether or not the company is registered under section 12 of the SEC act? is part of the debate.
But the practical answer is this: The SEC doesn't enforce it.
The material question IMO is whether someone can trade on insider information. The answer to that is "no" and I base that on the general sections that prohibit manipulation, such as Reg. Sec. 10(b)-5 and similar sections.
It's sort of like asking someone, "is it ok if you don't report options trades on your Form 1040 because it's not on the 1099B?" The answer is you report all income, but the bottom line is that many people don't know this or don't think about it when they are rushing to file. Of course, some intentionally don't report and we can all say <together now>, "that's wrong."
It's kind of common sense in a way but the bottom line again is that the SEC doesn't have the manpower to enforce so I'm willing to bet that there is plenty of trading by pinkie pennies insiders with insider information.
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