Earlier quoted context omitted.
I’ve read a couple descriptions of the internals of git that don’t disagree with the design of SVN, so I’m not unsure why you can’t theoretically check out a single subtree. Either the documentation is too hand wavy or some implementation details have blocked that possibility.
Are you aware of git-subtree?
Also: > The responsibility of not mixing super and sub-project code in commits lies with you.
Is a lie. The responsibility of not mixing code lies with every member of the team. Does the author work alone?