> Seems like this is proof that it's not.

How so?

In order for this MITM to work, you must hand over the encryption certificates, so to block MITM, don't expose your private certs.

> I've never understood how/why encryption is the answer to man-in-the middle.

No encryption = Bob ---> Attacker (reads files) --> Alice

Encryption = Bob ---> Attacker (cannot read files) ---> Alice