More ambiguity on their part. They don't actually say "It is an SBR if".
(2) When a weapon provides surface area that allows the weapon to be fired from the shoulder, the following factors shall also be considered in determining whether the weapon is designed, made, and intended to be fired from the shoulder:
(iv) Whether the surface area that allows the weapon to be fired from the shoulder is created by a buffer tube, receiver extension, or any other accessory, component, or other rearward attachment that is necessary for the cycle of operations;
The way I read it, if the buffer tube is necessary for the operation of the pistol, it is not a brace.
AR pistols with buffer tubes are still being sold without requiring a tax stamp, which leads me to believe they will remain legal.
What the statute says is when there is either
Other rearward attachment
The agency shall consider, which ordinarily means the agency is compelled by law to do what is stated in 2 (i)-(vi).
When you read 2 (i)-(vi), there is no punctation or other grammar or words that indicate the agency has the leeway to only consider one of the six factors. This is because the statute includes semicolons at the end of (i) thru (iv) the statement at the end of (v) is AND, which conjoins (i-v) with (vi) which ends with the only period in sub-para 2.
In short, sub-para 2 and (i) thru (vi) is one sentence in which all things in (i)-(vi) must be be considered in totality for ATF to make a decision.
That the agency doesn’t even understand the mechanics of the rule they wrote indicates an astounding level of incompetence.