The GP comment you were referring to recommended streaming Netflix in 1080p to, assumedly, compare to streaming games in 1080p, too; not to compare 1080p versus 4K. If you can stream Netflix in 1080p, there's not much additional strain on the network to stream games in 1080p.
Side note: Stadia also supports streaming games in 4K, which will have a relatively equal quality to streaming a 4K movie, for the same reasons. That is the result I see while streaming a movie and a game side-by-side.
Netflix in 1080p is nowhere near the quality required to play games in 1080p. It is heavily compressed, whereas games need crisp precision because it contains a lot of text that should be rendered precisely, as well as having pixel perfection for a lot of in game elements. To convince yourself look at Twitch, which has higher bitrate than Netflix for a similar resolution, and realize than even that is far from good enough to be playable.
Also video encoding must be done in strictly realtime for gaming. Twitch also prefers realtime for communication but not so strict. Netflix pre-encode their videos so they have significan quality advantage even in same bitrate. So Stadia needs more bitrate than Netflix for same quality.
Side note: Stadia also supports streaming games in 4K, which will have a relatively equal quality to streaming a 4K movie, for the same reasons. That is the result I see while streaming a movie and a game side-by-side.