> There is no requirement to invite someone someplace just because you are going there yourself.
That wasn’t the effect you were demonstrating above. It was more “you have to invite [friend A] if you invite [friend B] from the same social group, unless you want [friend A] to think that you value them less than [friend B].”
Which is sometimes obviously what you want them to think—if you have known [friend B] for years and you’ve just joined the group that introduces you to [friend A], then hanging out solely with [friend B] (in a way that [friend A] will find out about, like by inviting them in front of them) is a way to assert that you’d like [friend A] to get to know you better, because you’re still not at the level where you feel comfortable adding them to your more close-knit friend group yet.
The problem comes when you’ve just joined a group and so only know everyone equally well (i.e. not that well), and yet only invite some of the group members somewhere anyway. This reads like the signal above, but without a justification: the people you chose to invite out did nothing to “deserve” the choice, and there’s no obvious thing (like more shared conversation) that you’re signalling that you’d like out of the people you didn’t invite. It can seem like you’re just sociopathically playing members of the group against each-other to win your favour.
But that’s really the only scenario where this applies: when picking a subset of acquaintances, from a shared group, to do something with that is usually reserved for friends. (It does come up a lot, because a “pool of shared acquaintances” describes a school pretty well. But it’s not nearly as much of a “thing” outside of that.)