If there is no one else on the network between you and the server (like on your wired home LAN with no one else on it), you’re good, regardless of HTTPs.
It’s not enough for the network to be untrustworthy for MITM attacks, they have to use a certificate signed a by root certificate that your computer already trusts.
Organizations with those IronPort gateways use device management and Active Directory policies to pre-install a root certificate into your OS. The IronPort decrypts the original server then re-encrypts it with its own certificate to your computer.
If you used a non-organization managed device on those networks, it would show big scary warnings before letting you visit any HTTPS site that the certificate issuer is not trusted by your computer.