Also lived reality wise, for your average priest your classic highly approved 20th century moral theology manual had much more practical weight than Casti Connubii, something he might very well never even read or only ever heard a summary. And he acted quite rightly doing that.
Also it misunderstands
obsequium religiosum, a parish priest could perfectly say "Well the Pope says NFP can be justified, so I won't refuse absolution to anyone on that basis, but I still find it unnatural, untrusting in God and a bad idea all in all, and I am totally free to think and say that", not to mention pre-modernity.