But they are not walled gardens.

I would posit that any device that can't be jailbroken is a walled garden. Whether the wall is made of an app store or an operating system, it's not yours if you can't do what you want with it.

But corporate devices can boot any OS you might like.

Sure, they have MEs that maybe you can't disable, but you can firewall them.

Server kit is just not like consumer kit. Even laptops are [still, for now] a lot better than smartphones in this regard.