The Google fonts are used for the flat theme, I believe. Also, the speed gain from using a CDN on a mobile device over a mobile network is tremendous. The Arduino web server is absolutely bare bones with no caching logic. Google’s servers manage this load a lot better. Also, multiple mobile apps might share the same CDN link as a result no download is ever done for the largest files of this mobile app. The app will almost always be accessed with the internet active. Even if you don’t have internet, so long you visit the mobile app once with internet, it will work.
If I am saying something that is incorrect please let me know. Also, show me some speed tests/benchmarks please. Everything I have read/seen suggests large libraries like these should always be on CDN.