This is confusing, but we're definitely not passing it. This spec was added recently with the addition of a "buffer" kwarg for Array#pack.
It appears from the result that we are supports to take the "@4" as being from the beginning of the given buffer, leaving the existing four bytes in place and writing the new four non-zero bytes at offset 4. Instead, we append the nulls and the new values.
From test/mri/ruby/test_pack.rb in test_pack_with_buffer: