I think you are looking too hard for a secret purpose. I think they just want to keep making bets on controlling the social network space. They see Twitter fail

I think you are looking too hard for a secret purpose. I think they just want to keep making bets on controlling the social network space. They see Twitter failing and it seems like a great time to try and snag the market. They use ActivityPub as a selling point and Instagram users as an existing network and hope to replace Twitter. Instagram still seems pretty popular but they are clearly missing some part of the market with a text-based network so they are both expanding and diversifying. Facebook is already circling the drain (although it will probably be circling that drain for a decade) and they don't know if Instagram will continue to be popular. I'm sure harvesting data for LLMs is a great side benefit, but if that was their main goal they would probably just run a crawler. They could even crawl ActivityPub without having their own instance if they wanted. It is a public API. They would just miss out on private posts which is probably a small fraction.