100MB? Assuming they're using GZIP, that's normally a 10% compression ratio, leading to about 1GB of actual data. Let's assume this data is stored as 8 bits, that's about one hundred million characters of code! Maybe what they're doing is downloading the entirety of Windows XP (roughly a gig in size), running it in JavaScript and then rendering the page in Internet Explorer within their version of XP.

Yet, it works as expected without the polyfills.

Right. But he's suggesting that if they don't recognize the browser that they inject as much polyfills as they can so everything works no matter what.