Here's the problem with that survey. Many people who are asked in a survey if they are postmodern and/or secular humanists don't know what you are talking about. They are just hard-core liberals who don't believe in God and, without knowing it, have a postmodern view of life (i.e., there are no objective truths and reality is what you want it to be).
Postmodernists and secular humanists in America, even those who don't know they fit into those categories, probably comprise over 25% of the population. And that percentage rises significantly when you include the enablers and less serious believers.
I agree. It’s telling when pollsters do a survey and IIRC only ~7% say they are saved by God’s grace, with the rest saying they think they are pretty good people or are saved by good works or that God saves everyone. It’s impossible to accept the notion that America is ~70% Christian with results like that. Most people are secular humanists or postmodernists whether they’ve ever heard the terms or not.