I'm not going to say Redux-zero isn't useful, but if you're just calling some functions that update the state, how is it even related to Redux?

Redux is all about reducers. "Redux" with "no reducers" is an oxymoron, like "pottery without clay" or "cow herding without cows". Redux is a few tools and an ecosystem to support you in handling your state with nested reducers. The value is the predictability you get from using reducers.