Sure there is a source. No one thinks the writer made it up. Its on the writer, if youre trying to be fair and accurate, to do better. Much better.
I don't see what the writer did wrong and why you think it's so unprofessional. He presented the view of the source, and made clear several times that "the source said this". He reached out to the team to get their point of view, to poke holes in what the source said, they didn't want to comment at all. Apparently several people he knows with the team didn't want to even deny it. Obviously he thought the source was someone trustworthy and in position to know things or he wouldn't have written it at all (I am giving him and Pitcher List the benefit of the doubt that they aren't just some clickbait outfit from past viewings of each). And like anything else, everyone is entitled to not believe a word of it. But really the base stuff seems pretty believable, one can pretty easily strip out the slanted opinions of the source and ignore those.