[Thorrez]

> How the receiving party can be sure the key was not switched by the all-mighty middle man who can control everything?

From the article:

> Participants will also see the meeting leader’s security code that they can use to verify the secure connection. The host can read this code out loud, and all participants can check that their clients display the same code.

Obviously the vast majority of people won't do this, so the vast majority of people won't be fully protected against active MITMs. But the potential of meeting participants doing this will discourage attackers in many cases.