I suppose I just cannot imagine a situation where a rich interactive web app needs SEO that is not solved by having a separate landing page (and such solution not being way better and less effort than alternatives).
React has enough flexibility to be added later on to a project and not take over the entire web page, but only the parts it manages. It can act as a separate island or series of connected islands. So if you start thinking you dont need react but then later decide you want to add it to your non-react site, it should still be easily doable.