MitM with regard to unauthenticated clients I'm trying to understand how man-in-the-middle attacks are prevented during key exchanges, and am confused by this Wikipedia statement:

Use of mutual authentication, in which both the server and the client
validate the other's communication, covers both ends of a MITM attack,
though the default behavior of most connections is to only
authenticate the server.

Is that saying the client authenticates it is really receiving from the server, but the server doesn't care who is sending?  Doesn't that mean the server might be talking to an eavesdropper who inserted himself in the middle and the eavesdropper can't decrypt server responses (which are simply forwarded to the client) but can pretend to be the client and see what the client is sending?
but can pretend to be the client and see what the client is sending

No, the eavesdropper can't see what either side is sending except the ciphertext and size of the ciphertext.
To eavesdrop on any connection where the server is authenticated, the eavesdropper has to be able to convince the client that he is the server, which the eavesdropper cannot.

Doesn't that mean the server might be talking to an eavesdropper who inserted himself in the middle

Any internet middle box can do that.

and the eavesdropper can't decrypt server responses

True, and the eavesdropper can't decrypt client requests either since the eavesdropper doesn't have the session key.

but can pretend to be the client and see what the client is sending

Again, the eavesdropper doesn't have the session key, he can't do that.