How long could one actually be even moderately active LDS (past age 8 - obviously a four year old can be an active Mormon all his life and still not know how to spell it correctly) without knowing at least the basic beliefs? How many who have been inactive so long as to not know central concepts still claim the religion? We're not talking about [insert random word or phrase not necessarily having any logical connection to Christianity] Baptists here.
In this context, claiming the religion is a cultural identity thing, not a religious thing. Focusing strictly on the LDS example, generally the sort I'm talking about were
never really even moderately active. General template is this: parents aren't active in the church, don't teach their kids anything, but they consider themselves Mormon and they want their kids to be Mormon. So they bring the kids to church long enough for the kids to be taught and baptized, then drop off again. Generally the kids will then attend rarely-to-sporadically until they're in the midst of being a teenager, then rarely-to-never. If you ask them as an adult what religion they are, they'll say Mormon, but they know or remember almost nothing about the basic doctrines.
Replace LDS/Mormon above with Catholic or Protestant, and I've met many people that fit the description there too. e.g. People that say they're Baptist, but have a distinctly antitrinitarian or modalist understanding of God, etc.
Not following isn't the same as not knowing. And denying something that's in the Bible while believing that the Bible itself is the word of God is still somewhat contradictory.
I don't disagree in the slightest. I mentioned Cafeteria Christians for a reason. I've heard all sorts of mental gymnastics to (attempt to) rationalize doing exactly that, from people of multiple Judeo-Christian faiths.