I don't believe that's the case. Fr. Marx was teaching NFP before Humanae Vitae, and before he founded HLI. But at that time there was still the concept of "grave reasons," it wasn't promoted as a lifestyle.
There was an ineter-regnum when Fr. Marx was forcibly committed to house arrest at his Benedictine order, and HLI was taken over by some liberals. These traitors temporarily agreed to withdraw the severe criticisms that Fr. Marx had made of the bishops' sex ed programs.
But after a year or two, there was a counter-coup, the liberals were thrown out, true followers of Fr. Marx were installed in all the key positions, and the first major statement by the new administration was that they were withdrawing HLI's temporary disclaimer of their opposition to sex ed programs.