Content note: Subtitled swear words, talk about sex, and child rape.
"His shelf breaks," in the video's title, refers to the Mormon adage of "putting a thing on your shelf" to mean no longer contemplating something that's causing you to doubt the institutional church's claims or good intentions. Ex-mormons talk about breaking shelves to mean that something finally pushed them over the edge.
A lot of shelves have been breaking since their church recently published its essays, admitting to things like how Joseph Smith preyed on a girl "a few months from her fifteenth birthday," and those essays getting front-page, mainstream attention in places like CNN and the New York Times. Even though the essays still hide or obscure some things, and always phrase stuff in a way that creates the most ambiguity and casts the best light on the church, there are a lot of real people who are finding this stuff out right now and having the same reaction Memetic Hitler did in that video.
So you could say it's funny because it's not funny!
May whatever gods hear their prayers be merciful to them, and help them and their families escape.