The evidence is a combination of two things: Tristan insinuating they did (note, not claiming but suggesting or implying), and their own statement saying "While unlikely, we cannot rule out that de-identified data derived from their usage of our products helped improve our models ."
The latter is being used by everyone as a sign of guilt, but it's clear that they are making a legally-safe statement since promising a forensic level guarantee that nothing Tristan has every typed into ChatGPT has ever made its way into any training data is a massive claim.
It's also a ridiculous expectation since tons of researchers use ChatGPT or Codex and many have "Use this data to improve" setting on.