This here is where the linguistic descriptive abilities of the English language fails.
Diffie-Hellman is resistant to mitm iffMITM if an out of band-of-band third party entity is able to help in distributing keys and/or verifying the identity.
You'll find in the literature, what it mostly defines is a connected web of trust surrounding the identity of the recipient or, in most cases, the person attached to the key or certificate.