And if she were acting in her own rational self-interest, she would do the same, since her well-being (and in fact, as neatly illustrated in the comic, her very life) depends on the well-being of the group.

This assumes perfect foresight. As can be seen from the history of robber barons and the legacy they left, it generally did work out for most of them, so they were correct in their choices focusing on self-interest. Not since the French revolution has any significant number of rich assholes faced significant consequences for their choices in placing their personal welfare above the group.

But that has nothing really to do with rational self-interest as an idea.

But that’s the stance that proponents of ‘rational self-interest’ have settled on. It’s not just a mindset, it’s an ideology. The mindset you have in mind may make sense, but the ideology it has become does not, and that is what people are making fun of.

The issue here isn’t her being on social security, it’s her arguing against its existence because ‘Nobody should need it’.