As the article you quote says right afterwards, "Stimson's ethical reservations about cryptanalysis focused on the targeting of diplomats from America's close a

As the article you quote says right afterwards, "Stimson's ethical reservations about cryptanalysis focused on the targeting of diplomats from America's close allies, not on spying in general." "Gentlemen do not read each others' mail" is essentially about respect to your in-group peers. That notion did not and does not apply to mass surveillance - gentlemen of the time would not consider the "common rabble" as gentlemen. Your point about mutual respect is quite touching; perhaps it should be phrased as "some people will, in fact, read others mail" (without each others') because the key point is a lack of mutuality, an asymmetric expectation of unequal involvement in decisions - it's less about people reading each others' mail but more about one tribe choosing to read the other tribe's mail for Proper Reasons, decided and approved by the tribe who's doing the reading.