A million people will likely have more land each. They’ll be able to have gardens and maybe even raise their own animals.

Is that a net environmental win? Rural housing is much lower density than city housing -- so if these million people each had one acre plots and devoted half that land to gardens (which would feed roughly half a person for a year), they'd have a population density of around 0.4 people per acre (assuming 2.5 people per household). Most of the rest of the land would be covered with houses and landscaping.

And that's assuming that each of these people are willing to work a half acre backyard farm, which is a lot of land to maintain as a hobby.

So they are consuming 400,000 acres (625 sq miles) of land, with half of it replacing dedicated agriculture, but the other ~300 square miles is going to low density housing. (which is impossible to serve effectively with transit so everyone has to drive to go anywhere)

In contrast, San Francisco houses 800,000 people on 49 sq miles of land. (which doesn't include the roughly 250,000 workers that commute there every day)