I don't think there is any language in the rule which specifically says that they will never come after anybody after they register the SBR. Even if there WAS such language, I trust the AFT as far as I could throw your average hippopotamus.
I mean, you ARE admitting to a felony, which is why you need "amnesty". Normally the process is submit Form 1 or 4, wait (a long time), get approved, THEN build or take possession of SBR. With this thing, you're admitting to having an unregistered SBR BEFORE registration.
For this, and several other reasons, biting the cheese on these "free tax stamp" SBR's is a terrible idea IMO. YMMV.