Compare two packages which are identical except for package name.
What did you expect to see?
apidiff should report a change if the package name is different. I'd argue that this is always an incompatible change.
An interesting question is whether changing a main package to something else is compatible. From the perspective of importing packages, it's a compatible change, since main couldn't be imported before. However, anything that depends on a main package being built as a binary will break.