Using AES in ECB mode is clearly a bad choice, but honestly it's not that horrible for high entropy data like compressed audio/video. I'm sure someone could prove me wrong one day, but it seems hard to extract any useful patterns out of compressed audio/video. It does check the box of "uses encryption" for regulatory reasons (while missing the intent). It's pretty egregious considering how easy this is to get right.
The 128-bit key is not inherently wrong if they were rotating these out during the stream. That being said, there's no reason not to do it right and use a mode like GCM with a longer key - most hardware supports acceleration for AES-256 these days. It can actually be slower to use a 128-bit key on 64-bit systems.
While I respect the decision not to disclose the waiting room vulnerability, it's pretty obvious what's going on given the context. They probably shouldn't have mentioned where the vulnerability is.
I'm honestly surprised anyone with technical knowledge thought that Zoom was actually doing end-to-end encryption given how the software works. All of the video transcoding/downconversion is clearly happening on the server. Your client is not sending multiple compressed streams for varying connection bandwidths. That's the main reason a lot of people like Zoom - it actually works well with dozens or hundreds of participants.