In the US perhaps. The EU is aiming by 2020 to have 100% of homes have at least 30Mbps+ available if they want it, and 50% of homes actually using 100Mbps+ (https://ec.europa.eu/digital-single-market/en/broadband-stra...).

20GB in an hour is about 44Mbps, so a 100Mbps connection is probably a good goal if you want Stadia to work well.

I'd expect high-speed connection uptake to be quite a bit higher amongst their target market, so over here betting than the majority of gaming users have 100Mbps+ at home isn't that crazy at all imo.