The proof is that Mastodon and his server were implemented in Ruby. It doesn't exist in another language. And it's the same language that he uses at his day job

The proof is that Mastodon and his server were implemented in Ruby. It doesn't exist in another language. And it's the same language that he uses at his day job and according to him, it's part of what compelled him to work on it at all. The fact that he built it and maintained it is the proof for the claim. Without Ruby it simply doesn't exist, so it doesn't matter how much hypothetically better your favorite language is.