Posts from people you follow appear in your sub feed based on my observations. Doesn't matter if you subscribed to the magazine in question, I assume "following" is just subscribing to a user behind the scenes. Given a magazine is just a special user account iirc.

I haven't noticed this in my own feed yet, but it's also possible boosts of users you follow are also shown to you.

So it might be worth checking who boosted that thread in your image.

Edit: Doesn't seem to be the case, no, you're following neither the creator nor booster of the post.

Have a look if you don't have an accidental subscription to kbin.social.

I had a similar problem. That seems to be a connection point for federated posts and makes all of them appear in your subscribed feed. It does not appear on your magazine list.