I suspect that people imagine Zoom getting a privilege due to their inartistic value but I don't think that this is the case. I think that it's just a business and there's no reason why you can't have it with Apple too.
I imagine, engineers in Zoom needed this to accomplish something and they told the management. The management contacted Apple management and explained them how this could work well both for Apple and Zoom and promised not doing nasty stuff. Apple trusted Zoom enough to open or implement that feature for them. Maybe Apple have seen the the need of this API, developed it and contacted Zoom to give it a try.
Why wouldn't they open it for everybody? Probably edge cases that can be worked out on 1:1 partnership but need good design when open to everyone.
It's just business. What would have annoyed me though is if a competitor like Skype request access in similer terms but got denied.
For some reason, the narrative on HN and similar tech heavy forums require us to pretend that business relationships don't exist or that business relationships are dirty elitist entities that you can't have. A lot of people can be surprised how much you can have by just asking to have it or proposing something that can work for everyone. Maybe that's the dirty secret of the non-technicals :)