The neoconservatives seem to have accepted that Iran is intent on becoming a threshold nuclear power, but not actually developing a nuclear weapon. So the language has been stretched to say that merely having the ability to make nuclear weapons is unacceptable.
It is worth noting that having the ability to make nuclear weapons is not contrary to the Nuclear non-proliferation pact. The following countries are nuclear weapons-capable but are signatories to the NPT and have elected not to build nuclear weapons:-
There's another passage in the article that is worth noting:-
Any doubt that this is the intention of the backers of this approach was removed back in March, when the Senate was considering new Iran sanctions. Senators Joe Lieberman (I-CT), Lindsey Graham (R-SC), Richard Blumenthal (D-CT) and Bob Casey (D-PA) offered their own "no containment" language to the sanctions bill and the Senate moved to quickly to accept it.
However, amending a bill once it is already on the Senate floor requires unanimous consent and one, and only one, senator objected. Rand Paul (R-KY) said that he would oppose the containment clause unless a provision was added specifying that "nothing in the Act shall be construed as a declaration of war or an authorization of the use of force against Iran..."
That did it.
Neither the Democratic or Republican leadership would accept that (knowing that AIPAC wouldn't) and Paul's objection killed the bill, for the time being. In other words, the purpose of "no containment" language is precisely to make war virtually automatic. Because Paul's provision would thwart that goal, it was unacceptable.
I know that I'm supposed to hate Rand Paul but his persistently principled stances at times make this decidedly difficult.