We don't know for sure what React-related patents Facebook really has or if they would claim any other libraries infringe them -- or whether those patents would stand up in court given prior art. There are vdom-like approaches that predate React.

That said, I would expect that vdom libraries like Mithril that work in a very different way than React would be less likely to be claimed by Facebook to infringe a React-related patent than, say, Preact which tries to duplicate the React API.

Of course, this is all speculation with a lot of unknowns. It all is another example of why software patents are very problematical ways to "promote the Progress of Science and useful Arts".