It seems reasonable for Zoom or any app to know whether its window has focus. That doesn't imply spying on anything else you're doing.
For example:
1. Zoom knows it’s not focused on Bob’s machine, and notifies Bob that someone has begun sharing their screen.
2. Zoom knows it’s not focused on Bob’s machine and notifies Sally of this.
Scenario 1 seems acceptable and helpful. Scenario 2 is invasive and unnecessary.