How do you know that data is authentic?

How else can you be sure the content is valid though? Lemmy posts are sent as Announces from the community, not from the actor posting the post. The community could just fake a post if you just trust it.

That’s not true, you know the data hasn’t been tampered with because of HTTP signatures. The fediverse isn’t just “yolo, just trust me bro”. There are some parts that are a bit more towards that side (particularly with groups on Lemmy) but I don’t think this should be.