I think the goal here is to build a free data harvesting machine for building LLMs. (and also a shameless plug: I featured this article in my last newsletter https://bigtechdigest.substack.com/p/most-read-tech-articles... )
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.