there is no reason to think that the land would be deforested, nor even that it would not have been deforested for some other reason, if everyone were vegan.
what would change if everyone were vegan? what would change to MAKE everyone vegan? you can't actually know what that world would be like. you're guessing.
the nature article only talks about the impacts of producing the various foods in the current circumstance. if people shifted to eating less meat, then circumstances would be different, but it's not clear that the environment would actually benefit at all.