The final sentence of 7-5-12 is superfluous, it does NOT contradict the first sentence of the rule, but I do believe it creates confusion in the interpretation of this rule.
I don't agree that a clearly worded, not withstanding the multiple negatives, sentence in the rules is "superfluous". That last sentence without any ambiguity defines what the team A linemen must do to gain the advantage of the expanded neutral zone. If he's not blocking, then he is restricted to the original NZ and does not get the benefit of the expanded NZ.
I would agree that we could use better English in that last sentence:
From the original negative text: "An ineligible is not illegally downfield if, at the snap, he immediately contacts a B lineman and the contact does not continue beyond the expanded neutral zone".
To an improved (IMO) positive text: "An ineligible within the expanded neutral zone is illegally downfield if he is not blocking a B lineman."
Or maybe some other thoughts on how to best reword the intent of the rule here?