Since Mormonism distinguishes between doctrines and relationships, I don't find it disingenuous at all.
For example, the doctrine of infant baptism is something that we believe God finds abominable. The doctrine of original sin, as well. Additionally, the idea of an eternal hell. All doctrines taught by various churches in Joseph Smith's time, and refuted in the doctrines of Mormonism.
Despite this, we believe most folks are doing the best they can with what they know. Most of my friends growing up were not Mormons (I was raised in Texas, Wisconsin, and Virginia). Far be it from me-- anyone, really-- to question the authenticity or sincerity of someone else's relationship with Christ.
What we question are doctrines. Not relationships.