I went down this rabbit hole a little further, this time on the correct site asking the fine folks over at TMDB if they wouldn't mind adding "Ocean's 8" to the "Ocean's 11" Collection?
I was then invited to read this previous content report:
Hi! The collection was locked to prevent users from adding the spin-off to the collection: Overview - Collection Bible β The Movie Database (TMDB)
So, no, it won't be added, but at least now I understand the rationale behind such a decision, and it's a pretty thoughtful one. The way they're using "Collections," they only consider them to include true movie sequels. Their "Collection Bible" explains it all, and foreshadows "a proper 'Universe' feature in the future."