They are on Amazon and I think without CDN, since I got the same IP address from Australia and the US. They are also enforcing HTTPS so they cannot take advantage of local ISPs caches.
But as I said, the Amazon traffic cost is less then the wage of one employee so it is manageable.
The download quantity is probably lower then I would expect seeing that since March 2015, there have been 11 versions released, and for each version, there are separate downloads for Windows, 32 bit Linux, 64 bit Linux and MAC. I am not a big Arduino user, but I have probably downloaded 3 or 4 times over the period. A keen user may have downloaded all 11 releases for multiple platforms.
It is definitely not the case that there have been 10 million new users over the year. Perhaps 1 million new users. Basically every time someone buys a cheap $5 Arduino clone on eBay or Aliexpress, they are going to download the latest development kit fron Arduino.cc.