But why is push vs pull the definition of reactivity?

I suppose we can say that there are different kinds of reactivity. Signals is one kind. Observables à la rxjs is a different kind (the whole model of programming with rxjs was referred to as "functional reactive programming"). Observables are push-based. Signals, as I heard, are a more complex primitive, which, under the hood, is push-pull.

React's reactivity model may be crap; but this doesn't make it non-existent.