Tl:dr: Perhaps I am just uninformed on the issues, but I don't see what the big deal is with a company like Meta joining the fediverse. If anything, I think it is a very good thing, as it puts more attention and dev time into making it a more functional and better place. So what is the issue?
I personally joined the fediverse because of the structure of it. It cannot be owned by a single company or person, and there will always be a lot of diversity and customizability to how you interact with people and control your data. If people don't like a particular platform, they don't have to use it. They can join platforms that defederate from areas of the fediverse they wish to stay away from. That cannot and will not change, just because of how it is built from the ground up.
So enter Threads, and I am starting to see a lot of fear and concern over this. I don't get why this is. It's not like the fediverse is owned by Meta now. They are just one more player in hopefully an ever growing list of major organizations joining the fediverse. Ideally, I would like to see as much of the internet using the fediverse as possible. It is how the internet should work overall, and I think the specific issues with a company should not detract from the benefits of them joining.
However, maybe I'm missing something. Is there good reason to be concerned or warry of a company like meta starting an app like threads? What are the major drawbacks and concerns here? Is it being blown out of proportion, or is there actually something to worry about?
Same reason we haven't "proven" that bigfoot, the loch ness monster, or jersey devil are "bullshit". In order to prove something, you need to find evidence for it. If there is no measurable evidence for something, that does not disprove it, it just does not support it.