Putting “here’s how we changed on a previous product” doesn’t really need to be on a small corporate site. It being a sort of investigation fits better on GitHub where fewer people will accidentally stumble across it from their current product.
Also:
* it's mixed up with all their other marketing material for this product.
* it's in their current product's repository
It's more likely people will stumble across it when looking at their product.
As I said in my edit. The reason for it being on their GitHub is to increase the likihood of GitHub stars which they use as social proof.