Ada array vs C pointer (call by reference)

In any case, there is nothing related to "portability" in
Interfaces.C. The types defined there are exactly as non-portable (as
far as interfacing is concerned) as any other type in Ada,

There is of course some added value in Interface.C.XXX - it is
documenting the intent.
Vendor supplied libraries with names like "Interfaces.GCC_C"
or "Interfaces.Visual_C" would convey that intent just as well,
but the source code would require modification to move from, say,
Gnat/GCC to some other compiler pair. Interfaces.C would not
require source code modification and in that sense is more
inter-vendor portable.