In the ASG there are some unused shots of what Earth looks like as well as a description. There is no green left. Period. Yosemite was given over to condos years ago, the falls are a sludge dribble and if you want to see trees you have to look at them in pictures.
That's what he meant by 'they killed their mother'. There is no plant life left except for the ocean and that's even pushing it.
So yeah, to people like Grace and the 'older' generation, Pandora is a 180 from where they came from.
He wasn't exaggerating, he was putting it in terms that Eywa would understand. I suspect also that Eywa would be able to see into Jake's memories as well and see where they come from.
It will make a bigger impact if we are able to see what Earth looks like compared to Pandora.
"You grew up hearing about it." takes on a new meaning.

/skipped eveyones posts sorry, its cause we are humans, our instinct is survival

If you have sunlight, filtered air and greenhouses, you have food, and life, if limited in scope.