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False.  There's no exclusion in Pius XI's teaching for "extraordinary circuмstances".  The burden is to explain with any particular practice how the primary end is not subordinated to the secondary.  That has never been done for NFP.  Closest anyone comes is to claim that this isn't the case if someone is "open to life" (code language for ... would not have an abortion if NFP failed).  But one could say the same thing of someone who would not have an abortion after a child is conceived because of a faulty condom.

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"Openness to life" is a post-conciliar concept introduced by Paul VI.  Right or wrong, it's not something that has ever been commonly stipulated by the pre-conciliar theologians or popes who affirmed the morality of periodic continence.
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The poster in question had the right conclusion but the wrong explanation, or at least a vague one.  What does "subjugate" mean?  In Casti Conubii, Pius XI taught that no reason excuses from the negative precept to not deliberately frustrate the marital act.  This is in reference to contraceptive behavior, as is made clear by his simultaneous affirmation of the intrinsic morality of sterile relations precisely because they entail a due ordering of ends.  At any rate, whatever the poster meant by "subjugate" it would not be correct to say that Pius XI made any exceptions to the necessary ordering of ends; he affirmed the morality of sterile relations (and periodic continence directly and specifically shortly after CC's publication) precisely because in such relations there was no intrinsic disordering.  It's not an exception, it's a completely different thing.
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The relevance of grave necessity is that grave necessity can excuse from affirmative precepts, like the precept to go out and multiply.  A separate but contributive point to the morality of periodic continence.

NFP is a piece of junk, "we must perform on this day of the month", takes all the fun out of it. More importantly the woman naturally has no interest during those days. At best it is an avenue of relief for the man. Practicing NFP is a sacrifice in and of itself. If their is danger of death to the woman, and the NFP method is used as an "escape valve", from total abstinence, maybe a 6 times a year, it likely is not a very serious mortal sin? Maybe a venial sin?  

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The popes and theologians who taught periodic continence (which is really its proper name, not NFP), Pope Pius XII most notably, were careful to point out that the mere presence of a sufficient reason is not enough to make its practice allowable; the motives of the couple must also be good (though if they are not, the unlawful use of periodic continence doesn't "become" contraception; it's a different type of sin, a sin against marriage as opposed to nature).  The Novus Ordo practice of imposing NFP as though it's something that should indiscriminately be used flies in pretty stark contrast against what the pre-conciliar authorities taught.