First, they can use the sandbox without going through the Mac App Store. Sandboxing is a good idea regardless of distribution method. That would improve security for everybody, without needing to 'let Apple get in between them and their customers'.

Second, Zoom already runs sandboxed for the other two ways you can run their client on Apple operating systems: the (iOS) App Store and the web. The Mac sandbox is the least strict of the three. So whatever they do, it doesn't seem to be hindered by 'severe limitations'.

I have yet to hear any feature that a legitimate videoconferencing application would need that would be disallowed by the macOS sandbox. Lots of other video chat apps are on the Mac App Store, like Facebook Messenger. Is the issue simply that Zoom is being sketchy and wants to continue to be sketchy, and sandboxing would not allow them to? That's not because the MAS is 'a trap'. That's its main feature.

The sandboxed APIs should be more than enough for a video conferencing client. (Which, after all, can run in a browser…)