Without auth, you know you're doing DH, you just don't know who you're doing it with. If somebody is passively observing/recording the traffic, they wont know

Without auth, you know you're doing DH, you just don't know who you're doing it with. If somebody is passively observing/recording the traffic, they wont know what the negotiated shared secret was. However, if they actively insert themselves in the middle, they can negotiate a shared key with you, and another one with the final destination. Then they can decrypt messages from you with the shared secret they negotiated with you, re-encrypt with the shared secret they negotiated with the other party and forward them on.