Having a certificate from a CA like LE proves that you don't have a local MITM. The MITM would have to also somehow get between LE's servers and the website in

Having a certificate from a CA like LE proves that you don't have a local MITM. The MITM would have to also somehow get between LE's servers and the website in order to get a trusted certificate. A self-signed certificate does not have that guarantee.