Downloading the game wasn't a problem for anyone, as far as I know the CDN held up fine. The problem was the steam storefront, i.e. actually buying the game.

This makes sense because there are several live service games on steam that have hundreds of thousands of active players - any update will put a large instantaneous load on the CDN as all the clients are forced to update at the same time, and this happens extremely frequently so they've had to get good at handling it.

What doesn't happen very frequently is hundreds of thousands of people trying to buy a game at exactly the same time, because most games have pre-orders.