Welcome to Shopify Community. My name is Olivia and I work here.
I appreciate your question and I'd be glad to share some insight. In fact, cdn.shopifycdn.net is an official Shopify CDN. We don't have public documentation available on this as much of this information is internal only, however, I can confirm that it is ours for you.
Read more about Shopify's Content Delivery Network through Shopify for Developers.
If you have any other questions I can answer, do let me know.