A lot of this seems like it comes down to how that initial pool of users is treated. If they show it to 5 users and his mom is the only one (ever) that likes it

A lot of this seems like it comes down to how that initial pool of users is treated. If they show it to 5 users and his mom is the only one (ever) that likes it, then it makes sense to show that content to people common to him and his mom. If the other users like it, they should distribute it more widely and maybe include users from other non-overlapping networks. That all seems good. If mom likes 90% of the posts, then show that 90% to the family and maybe they care and maybe they don't. If it gets engagement from other people, share it more widely. That seems like a good approach. The wrong way to do it would be to show it to 5 people and as soon as his mom likes the content, decide the content won't be interesting to the other 4 people they showed it to. I haven't seen any evidence of that happening, and I don't think the article points to any (anecdotal or otherwise).