It's just that moderators don't see everything. In this case whoever put the OP In the pool hadn't seen https://news.ycombinator.com/item?id=31570698.

It also sometimes happens that an article makes it into the pool before a different version of the same story gets posted or gets traction.

And sometimes (but rarely, I think/hope!) different mods put the different versions of the same story into the pool.

what do you mean by "included in the pool"? is that a HN thing?