150 million page views a month works out to 57.9 pageviews a second. So the vast majority of those requests must be JS, CSS, and static image requests, although 98.5% seems a bit off?
The 3900 requests per second are likely not constant. That number is probably during their busiest time during the day, so their infrastructure needs to support that kind of traffic. You're right, though, requests per second likely includes all file types.
