No, he's thinking of the requirement by Debian policy that all Debian packages must be able to be built from source stored in the Debian source packages.

Sanboxed programming environments are a workaround for the problem systems like cabal have where you don't have a consistent set of package versions that work together, you have all versions of all packages avaialble and then issues can happen (causing people to say words like "cabal hell").

If you use a consistent set of packages for all development (such as OS-provided libraries) this is not an issue.

I work on many (many) projects at a time. Sandboxes (in haskell, python, ruby, C, all my languages) would just add more pain to my workflow.

By "real GNU/Linux" I meant "not just android or sailfish"

OS packages are much nicer than cabal. You don't have to wait for everything to compile, and you get stable library versions that work with each other (no different-dependency-versions, no annoying sandboxing garbage).