It is even weaker than that. DKIM keys themselves can be stolen - most servers don’t store them in HSMs (and HSMs are also not infallible).

Or factored - Debian had a bug 12 years ago that caused weak SSH keys, a similar thing could happen to DKIM key generation (or has happened, but not yet discovered).

Some study showed many RSA keys in the wild had a common factor. A weakness of this family might be discovered with DKIM keys.

It is supporting circumstantial evidence, not proof of identity.