Google has 2.8k public repositories on just their main github account (github.com/google).
Even if they're not training on their closed source internal codebases (which they most certainly are, but fair point that they're probably not releasing the models trained on that data), it definitely seems like they have a lot of Go code in the training data.
Can't read the article due to paywall, but potentially ambergris? It's a form of whale excrement that washes up on the shore and can sell for ~$10k USD/pound
Even if they're not training on their closed source internal codebases (which they most certainly are, but fair point that they're probably not releasing the models trained on that data), it definitely seems like they have a lot of Go code in the training data.