If the company doesn’t keep them employed, voila they must leave. And presumably the company is not going to hire (or allow to live there) ‘undesirables’.

It's not that sort of company town. The Company is the town and the town is the The Company. They will own all the buildings and infrastructure, be responsible for providing all the services and as such get to decide who gets to live there and how, but they'll be doing so with a goal of running a profitable company/town. The people living there won't (necessarily) be working for the company. As long as you can pay, agree to live by their rules and aren't 'undesirable' in any other way you can potentially live there.

It’s the same thing, just with no customers being able to show up on site no?

Or do you think the company will allow randomly renting to non-employees in the housing? Or subletting or something?

The housing won't be (primarily) for company employees, but for anybody who can pay and gets approved. Most company employees will in fact probably be people doing maintenance and providing services to the residents, and they almost certainly won't be able to afford to live in the the town.