But note that on the scale of move as much data around as Google does, or even "the web as a whole", shaving even a few bytes off of every single gzip packet stream can still equate to significant network relief.
I suspect their advice is for their benefit, not other website devs. They can save a lot of space in their archive if everyone's pages were smaller. Nothing compared to better image compression though.
No - a few bytes on a web page are insignificant compared to the data volume of images and movies. This is all about getting pages to load faster on mobile.