Is it just me to whom it seems obvious why they've gone with ECB?
Zoom's design has a single key for everybody and for everything [ in the context of a particular video conference call ] . It's simpler and, to a layman, it sounds secure. [ We arguably contribute to this if we say e.g. "the key" implying it's just one thing when we mean something like a master secret in TLS used to derive lots of actual keys ].
Once you've committed to a single key ECB behaves exactly how you'd want.
You've got some audio, or video, ready to send? Just encrypt it with the key. Receive some encrypted data? Just decrypt it.
What happens if you have some network trouble briefly? Nothing, everybody decrypts whatever does arrive and maybe a few frames are missing.
All of the other modes don't work at all if you try to use them this way. They all expect you to have thought about the problem and track a bunch more state and then maintain that state despite an unreliable network and other issues.
Unless there is somebody in the room who says we can't do ECB because it's fundamentally not a secure choice, ECB is what you're going to get from this design decision.
And I've been in rooms like that as the only voice, or at least as the only person who spoke up. I've been in rooms where I was part of a chorus too, but as organisations get bigger and "security is everyone's problem" becomes a phrase people learn but don't act on, it gets lonelier.
Also, I actually can't even work out what a "correct" key rotation strategy could be for ECB with a variable number of parties all encrypting stuff at once. As a result it seems unlikely that Zoom did figure out such a strategy and then correctly implemented it. Instead it seems safe to assume there is no key rotation, everything sent by every participant for the life of the stream is encrypted with the same key, even though that's a terrible idea.