To be fair, Google Docs' core offering is live collaborative editing, not 'being a CDN'. Sure, there are ways they can support each at different times if they really wanted to, but it seems perfectly reasonable for them to say that when it gets to an N that they'd have to put significant engineering effort into supporting, that that's not what it was designed for, and that if someone wants CDN behaviour do that they can always export the document and put it on an actual CDN