You're right, and I actually make that point in the blog post! Reactivity is not exclusive to the frontend.
However, in the frontend, Reactivity is almost an invisible default. You don't opt into it, you just kinda have to when you use a modern framework.
In the backend, many people use Reactive patterns without even knowing them or thinking of them. But either way, if you do use them, you likely opted in, and it wasn't a "all code you write is automatically reactive" like in modern browser ui frameworks
I wish s-expression was a popular alternative. It's readable without the yaml issues.