If Mormons have higher fertility than non-Mormons then they’ll grow as a share of the population, even if the modal difference is less than one child. If the average Mormon family has three children when others have two they’ll increase their population share by 50% a generation, neglecting immigration and apostasy.
So? Human society and culture can change absurdly fast. If the evolutionary impulse to reproduce was that strong, you wouldn't be seeing places like Singapore with comically low birth rates.
What evolution compels us to do is have sex. We've just disconnected that from reproduction.
Re:mormons, yes they have a higher fertility rate, but it's also been rapidly dropping. And without taking into account conversions/apostasy the numbers are kind of useless, and taking them into account is difficult because it's just inherently hard to predict. As an ex-mormon I'm aware that the internet has caused some major problems with retaining members for the church, because information has become so much more accessible (much of it contradicting church teachings, obviously).
It compels is to do everything we do in life. There isn't a single trait ever measured in which the heritability was zero.
I'm an exmormon, birth rates are higher for Mormons, certainly, but even they've been obviously dropping. I had three batchmates out of 9 in the missionary training center with 10+ kids in their family. That used to be relatively common, now it's almost unheard of for newer families.