That's intentional, because landing is/was not supposed to be bootstrapped. It's original intent was just a playground like experimental, while the real deal is green.
However, for testing purposes, bootstrapping landing might make sense, so adding landing at least to PureOS' debootstrap is likely a good idea (adding it to Debian's version I am not sure about yet, due do landing's very generic name).
What usually happens is that you debootstrap green and then just add the landing repository (either replacing green, or on top).
Oddly though, that's not mentioned in https://tracker.pureos.net/w/development/pbuilder_environment_quick_setup/ even, so I think the docs need some update to reflect the current state of things (even if we add a landing suite to debootstrap later).
Ubuntu's scripts (/usr/share/debootstrap/scripts) are also in Debian. Would be great if the same were to be true for pureos since that would tickle to all downstreams and enable many more people to contribute without hazards.
@guido Ubuntu's scripts have a less-generic name though - that's why debootstrap in Debian has "green" and the Tanglu distribution names ("chromodoris", etc.) but not "staging" (Tanglu's equivalent to "landing") - I assume the same will be true for the generic landing name.
Meanwhile though we can update the scripts in PureOS, I need to revise them anyway, and add landing.