I think the author really means "library code" (i.e. code where the "user" is another developer). He also alludes to such code using metaprogramming which makes

I think the author really means "library code" (i.e. code where the "user" is another developer). He also alludes to such code using metaprogramming which makes internal type hints hard to implement. In this context, macros can be seen analogously to meta programming, or "infrastructure code" to use the terminology of OP.