Look a bit further where he plays on his phone. It also says in the article that it wasn't a problem on his phone. When you get lower latency through your phone, there's something wrong with your internet connection...
In the video it didn't say there wasn't a problem on his phone. It said it wasn't as present on mobile.
I was able to pause the video after he pressed a button and see no action in the game, so it's still significant. Just not as bad as 1+ seconds long.
The reason for this isn't necessarily the internet connection. There's both input lag and display lag going on simultaneously. And while testing on mobile he is using both a different screen, and a different controller, that are all wired differently.
I'm also not sure what the settings are for mobile. Stadia may be outputting at lower resolutions for phones to save bandwidth.