If I create a library I would usually make it to solve my own usecase. And if you mainly work with react-apps, and your plan is to use it in an react app. Why not also make the library in react?

In this case, I guess the developer wanted to use JSX to create it. And you could ofc use JSX without React. But most people are used to using it in react-context, and plan to use it in an react-app anyway.

So to answer your question, in my eyes it would be. Why not? If the devs plan on using it in react-app anyway, it's not their job to make sure it supports everyone elses usecase.