They don’t list their methodology, so it’s pretty much a made up number.
And yeah, back when you had to add the entire bundle to your account, you’d get a ton of crap bundle games that you’d never play. These days you can generally list the code for others to use.
It's definitely inflated but I'd say the real number has to be at the very least within 10% of that estimate (probably a lot more, but I've seen some AAA games at a 90% discount in the past) which is still in the billions of dollars, which is still kind of nuts