The problem he is referring to is not that facebook narrows down the audience based off of likes. His problem is that his mom, and consequently other family members, always immediately likes his posts regardless of the content which narrows the audience down to his family members instead of the intended audience. He wants Facebook to take into account that the person liking his post is his mom before narrowing down the audience to family members.

But the content is not intended for his mom, so he shouldn't have targeted it to everybody.

The post is intended for an audience including his mom (she interacts with his posts by liking them). The author's argument is that Facebook's algorithm is (incorrectly) assuming that her interaction is based on interest in the actual content, rather than her association with the author.

Sure it is. He even says that he later changes the settings to his mom sees it. He wants his mom to see it, he just doesn't want Facebook to get the wrong interpretation by his mom liking it.

How do you know his mom did not enjoy seeing what her son is up to, even if she doesn't understand much of it?

His mom is enjoying his content. However, facebook assumes that since his mom enjoyed his math or programming blog post, the blog post must have been baby pictures of his child uninterested to friends. Therefore, his friends don't get to see his programming articles.

Basically, your posts about work/hobby etc get penalized if your family likes them.