That's a long title, but let me explain.

The broad rule that manypkg warns about, is that anything in peerDependencies, should also be in devDependencies. This is reasonable (and imo also correct).

However, in a monorepo (i.e. something using workspaces), root level dependencies are used as a substitute for package level devDependencies. This keeps it easier to keep the versioning of those deps across workspaces simple, and avoids repeating them across workspaces.

Hence my feature request: You shouldn't warn about mirroring peerDependencies and devDependencies if it's already available in a root's dependencies. It should still do a version check ("External Mistmatch" rule), however.