GrapheneOS has plenty of evidence and they post it alongside their claims. They post it carefully though, and are willing to provide it to people upon request.
At the time of writing, I scrolled 4 posts down and found one. GrapheneOS are security researchers, so they often are a first party source. As for the attacks, they have plenty of evidence for their claims. They avoid giving any attacks more publicity, but they usually provide evidence if you ask.
Please provide a link to this post you found, so I can tell which one you think is a citation to a source. If you want some examples of recent posts that should have a source but don't, here they are: