The discussion of Zoom caused me to look into what FaceTime does. AFAIKT, there is no protection against MITM attacks by Apple against FaceTime. So in the end we have to trust Apple much like we have to trust Zoom. Of course one of those parties might be a lot more trustworthy than the other but the point still stands. For the sort of protection that e2e can provide the user has to have the ability to verify who they are talking to. Both Apple and Zoom are misleading by omission, one more than the other.

Assume you are on a call with three other people. Alice has a solid connection and you can deliver best quality video to her, Bob is on a low bandwidth connection from home and you will need a better compression ratio for him because he can't handle full bandwidth, while Carol is on a shit mobile connection that has high latency, low bandwidth, and drops packets. For each of these you would want a different encoding scheme and possibly very different packet sizes, but if you are sending it from your desktop at home you would basically be sending almost 3X the amount of data to handle the three different streams (plus the additional CPU for all of the different compression steps) -- Zoom sends one stream to their servers and then re-broadcasts at different compression levels and different packet schemes to give the best result for each user.

Justifiable, but in no way shape or form is this E2E encrypted and any company that makes this claim is committing fraud.

The tl;dr of "Zoom E2E Encryption Explained Like You're 5" is rather simple: "It isn't E2E encrypted"

You can't compress the ciphertext encrypted with typical encryption algorithms, because the output is indistinguishable from uniformly random bytes.

But you can losslessly compress data encrypted with Caesar encryption, because it's unbelievably weak.