Gemini is bad at coding, but good at writing. More human like
One could argue that Google Colab would supply the training data for better coding performance. I would argue that Colab is mostly used for non-complex (e.g., small number of variables) and self-contained (i.e., runnable in one page) code that can’t train a model for multi-folder and multi-page projects that rely on global connections, which real-life coding would often require.
You may well be right about Google not including the said data but your support for this point is weak.
Malicious acts from large corporations are certainly well known, but they're also grossly over-represented in the news and mindshare. Mostly things just work as they say they work.
I don't really know what proof you're looking for, it's well known that it's hard to prove a negative. I'm not suggesting taking Google (or anyone else) at face-value, but rather saying that when you think about T&Cs, laws, PR, etc, it's unlikely.